**PREFACE: For players already pretty familiar with LeBlanc's skills and know what items to buy should skip over to the sections on Spell Combinations, Laning, and Team Fights. I hope you find this guide informative.

LeBlanc is a fun character capable of some really flashy tricks that are simply amazing when you pull them off which is why I play her. I would not really recommend her for games you absolutely must win because there are other casters which contribute much more to team fights and are, in my opinion, better overall. For example, Kassadin has superior chasing and escape power, better AOE, and a more reliable silence (but cannot put out the same short term burst as LeBlanc). Another example of a caster I would pick over LeBlanc would be Anivia because of the additional utility she brings to team fights with her AOE slow, ice wall, heavy hitting single target nuke, and a stun that has the possibility of CCing more than one enemy and will not be stopped by other enemy targets(as opposed to ethereal chains which is not only narrower, but can be intercepted by a creep/tank/whatever). Other casters are generally better at farming as well. Finally I think a fed LeBlanc will bring less to the table than other stronger casters who are fed (eg, 20xSoulstealer Annie can wreck a whole team with aoe whereas 20x soulstealer LeBlanc has to settle for taking one target out). When without item advantage, the better casters also offer more utility in team fights than a LeBlanc without item advantage. LeBlanc's strengths over these other casters though is her ability to generally deal more burst in a shorter time, and keep her victim silenced while the burst happens. Also its just way more slick when you kill someone and then nonchalantly disappear. Way too cool!

Pros:
-Ability to chase down a straggler is pretty good. (But no sustained DPS to fully capitalize on chasing)
-Ability to escape is fantastic. (Double distortion + juking possibilities with the recall component of distortion)
-Great single target burst DPS. (Softer targets, such as enemy carries, should vaporize)
-Potentially capable of keeping a target silenced for a long time. (Typically 4 seconds of silence, up to 6 seconds of silence with the appropriate spell combination and CDR)
-Potentially capable of keeping a target snared twice, or two targets snared.
-Fantastic poking ability that is usually safe because of her silence and blink. ("Poking" almost is an understatement.)
-The ability to use Mimic to copy one of three skills makes her able to adapt better to different situations, and makes her a little more unpredictable to enemy players.
-Has really flashy combos, can execute somewhat safe dives with smart use of distortion.

Cons:

-Extremely squishy (collateral AOE even has the potential to really injure her. It is amazing to survive if you are CCed and focused)
-Fudging your mimic (casting skills in the wrong order) makes her really suck. Playing LeBlanc requires a lot of familiarity with her combos.
-Usefulness in team fights is typically limited to bursting one target down, then occasional nukes/snares/silence.
-Ability to silence is nowhere near as good as others with silence, because you can't just silence someone to stop something. (eg, if you want to silence Nunu channeling Absolute Zero it would take two spells rather than just one like it would take other casters; you might be forced to blow one of your cds that would have been better used to nuke an enemy DPS)
-Terrible farmer. (Last hitting will be your new best friend)

Masteries: I play 9/0/21 almost exclusively. I have toyed around with 21/0/9 and concluded that I'm not skilled enough to play a glass cannon THAT fragile. I've also experimented with 0/7/23 to get strength of spirit, but the HP regen didn't seem worth it given the relative safety LeBlanc can harass with.

Summoner Skills: I take either Ghostwalk with Flash or Ghostwalk with Ignite. Ghostwalk is the obvious choice. I find the toss up between Ignite and Flash a pretty tough one to call. Ignite is definitely great for sealing the deal when you lay down all the burst, but it also loses effectiveness as the game progresses. Flash on the other hand is useful in all phases. Unlike ignite it can be used both defensively and offensively, and when combined with Distortion and Mimic it gives you amazing chasing/escape power. The main reason I get flash is in fact because to execute super gimmicky dive/chases with three blinks. Flash opens the door to some interesting combos which will be discussed later. When LeBlanc's passive triggers, if you are not CCed in the split second that you are stealthed you can potentially flash away which is kind of amusing especially if you flashed into a bush or out of enemy vision, because it leaves your clone there to die alone. Finally the reason I recommend flash over ignite is because LeBlanc is so squishy, and having flash helps a lot in letting you live another day.

*I have a sneaking suspicion that ignite might actually be better but the allure of being able to pull some gimmick combos (as described in a later section) is just too difficult for me to resist.

Just a preface, I don't care too much to "optimize" my runes because I build them around the way that I play, and when I look at all the benefits the runes give they really pale in comparison to the benefit derived out of improving my own play. (ie, I don't really care that much about a bit of mp/5 from runes when I still have so room for improvement in mana management) That being said, I cannot definitively say that there is only one set of runes for the best LeBlanc (and if there is, sorry I cannot tell you what it is)

My suggestions:
Quints: Flat AP or Magic Pen. It's your judgment whether you prefer +14.85 AP or +5.67 magic penetration.
Red Marks: Greater Mark of Force (AP per level) or Greater Mark of Insight (Magic Pen). The reason I like AP per level over flat AP reds is because flat AP runes give +.59 each while the scaling ones provide +.1 per level, which makes them superior at level 6, which is when LeBlanc starts to become powerful. Again though, it is a matter of preference. Magic pen runes are also a viable choice for obvious reasons.
Yellow Seals: I find Mp/5 per level to be pretty useful, but I'm not sure how much the recent nerf to mana regen runes affects their viability.
Blue Glyphs: I like CDR/Level glyphs but flat ones are great too. CDR/level glyphs surpass flat CDR glyphs at level 13. The CDR comes in the most handy when team fights are more frequent because I find that in earlier confrontations using all your cooldowns usually results in a kill already. However there are definitely situations where flat CDR runes (total of 5.85% CDR with nine) will help sigil of silence come up JUST in time for a kill.
Really though, play with which ever runes fit your play style the best.

Passive: When LeBlanc's HP drops under 40% she stealths for 0.5 seconds and creates a clone that does no damage but lasts for up to 8 seconds. This is a pretty lackluster passive in my opinion, but it does have some interesting nuances. For example, if you are fighting someone and they are committed to fighting you or dying (and you are in the same situation), when this passive procs you can continue autoattacking them and alt+click your clone on a retreat path and the enemy will often chase the clone, which can buy you enough time for your Q to come up again to secure a kill. Obviously this is unlikely to work more than once, but even doing it once is pretty hilarious.

You may also have the clone tail you (or strategically block) when you run away too to certain types of skills (eg, Ashe's frost volley) or skillshots(eg, Morgana's dark binding.) You can even use it to scout around a bit or bait (eg, you are almost dead but managed to get away but your team is still fighting. Send clone in and maybe someone on the other team will get greedy and blow something on your clone). If it goes too far it will go back to you, like Mordekaiser's ghost. One important thing to note is that this passive will not pop projectiles like Shaco's Hallucinate, nor will it reset tower aggro off you. All in all it's a fun, interesting passive, but nothing too special.

(Q) Sigil of Silence
Your main nuke. On use it will hurt someone for 70/110/150/190/230 (+0.6 per AP) and put a debuff on them for 3.5 seconds. The debuff triggers when the target takes damage from one your skills, and upon triggering does 20/40/60/80/100 (+0.3 per AP) and silences the target for two seconds. Notice that the damage is heavily front ended so its not a huge deal to use it without triggering the debuff, especially considering the cooldown is an amazing 6 seconds. It has a range of 700 which makes it pretty decent and safe. For example Sion's stun has a range of 550, Annie's single target nuke has a range of 550, so this spell would be safe for simple harassment. You have to be smart when using it against people who have same or greater range than you (eg, Kassadin's Null sphere has 700 range too, Ashe's volley has 1200 range). Generally you would only want to go in for this kind of harass when you know their spell is on cooldown and thus no fear of trading hits, or if trading HP is really favorable for you. LeBlanc is definitely not made for battles of HP attrition. One final note about this skill, the AP ratios don't look phenomenal but you can think of this skill as effectively receiving +0.9 damage per AP, so long as you are able to trigger the mark too.

(W) Distortion
The amazing "blink." This is basically flash with a range of about 600 (higher than summoner spell flash which is only 475 I believe). It leaves a marker on the ground from the point of origin for 3 seconds which you may use the skill again to return to(I call this the "W-Beacon"). It does 80/90/100/110/120 AOE damage to the area where you land (about 250 radius; similar to Heimer's grenade but smaller than Morgana's tormented soil) and is on a 20/18/16/14/12 second cooldown. This move won't pop projectiles like a turret's shot so if you are diving and use distortion to get away, the tower projectile will still chase you. With enough AP you can use this to farm balls of creeps, but unless you are really fed you'll have to mimic this skill to get them all.

(E) Ethereal Chains:
The skillshot slow/cc. This is basically a skillshot with 950 range that attaches a chain to enemies which slows by 25% as long as the chain doesn't break. The initial hit deals 40/65/90/115/140 (+0.4 per AP), and if the chain remains unbroken for 2 seconds the target takes another 40/65/90/115/140 (+0.4 per AP) are snared for 1/1.3/1.6/1.9/2.2 seconds. For comparison, the chain does not nearly go as far as other skillshots such as Morgana's dark binding (1300 range), Amumu's bandage toss (1100 range) or even Lux's snare (1000 range.) The cooldown is fixed at 10 seconds across all levels.
This skill is great for keeping people off you, and obviously chasing/initiating as well. It's not phenomenal as an initiation tool because it only slows them, and after 2 seconds it only becomes a snare rather than a stun. However Sigil of Silence still triggers off both the first hit and the second tick, so if you nuke after the chain is attached, you can expect your target to be both snared and silenced after 2 seconds. When used defensively it is great too; melee characters approaching will approach more slowly and eventually eat a snare. If you retreat into a bush before shooting Ethereal Chains at an enemy chasing you, it is highly unlikely he will be able to dodge it. Like Sigil of Silence, the AP scaling doesn't look great but you can consider it to effectively scale for +0.8 damage per AP.

(R) Mimic:
LeBlanc's signature skill. This copies and enhances the last skill you cast, increasing the damage by 10/25/40%. The cooldown is significantly reduced as you level it up too. (40/32/24 seconds) Not much to say about this beside that it is a really fun skill that opens up a bunch of possibilities for LeBlanc.

Skill order
I usually skill R > Q > E > W, leaving one point in W until everything else is maxed out. If survivability looks like it might be an issue in the lane before I hit level 3, I put one point in W at level 2. I think other people like skilling R > Q > W> E, leaving only a point in ethereal chains until everything else is maxed. This is mainly a matter of my own preference, but I do it for a few reasons:
LeBlanc is pretty bad at farming, and the additional AOE damage that leveling up Distortion adds is marginal(only 40 damage difference between rank 1 and rank 5). Rather, the most compelling reason to skill up distortion is to reduce the cooldown time (20 second CD at rank 1, 12 seconds at rank 5). Ethereal Chains has a much better increase in damage (40+40 at rank 1 versus 140+140 at rank 5), and even more significantly, the snare increases from 1.0 second at rank 1 to 2.2 seconds at rank 5. A longer duration on the CC lets you not only get a few extra autoattacks, but also often allows you a little bit more time for Sigil of Silence (which has a really short cooldown) to come up. Most of the time after a full combo your target will be almost dead. The slight increase on the duration of the snare, in addition to the extra damage can easily make the difference between a kill and a near miss. Also, the cooldown reduction that you get out of leveling up Distortion is not so significant that you will have an opportunity to use it more than once during mid-game ganks. Ultimately though I think it is a matter of each player's own preference which to max out first between E and W. My playstyle with LeBlanc is to gank by getting in there, blowing everything on one soft target and getting out, which I feel E is much more useful for. The nominal extra damage that Distortion gets per rank is just not as good for this purpose, in my opinion.

This list of combinations is not meant to be exhaustive, just the ones I use the most and find to be simple and reliable.

[Q+R(Q)]Fundamental Damage Combo: Your staple DPS combo is simply Q + Mimic Q. Because you are putting the most skills into Q first, it will hit the hardest and naturally is the best candidate for Mimicking. This will also make them eat a 2 second silence, and if you can follow up with anything else, 2 more seconds. The cooldown for R is pretty short too so in a laning situation if it is obvious the enemy won't be dying soon I will just Q+R him as harass and try to get him low enough so that when mimic is back up I can execute a full combo for the kill.

[W + R(W)]Escape/Chase combo. This should be obvious. Cover long distances with this. When using W +W to kill a runner just make sure that you can (1) secure a kill with only Q and E and (2) you have an escape plan. If they aren't literally on the brink of death you have to make sure you can actually trigger the debuff your Q placed on the target. Many people underestimate the distance you can cover with this combination. Theoretically you can almost instantly cover 600 (Distortion)+ 600(Mimic)+ 475(Flash) of distance (total of 1675, approx. 1/2 the distance of Lux's ultimate pewpew lazer). Ethereal Chains has a range of 950, which can slow your target down enough too for you to nail them with Sigil of Silence, and if you were quick enough, the second tick of Ethereal chains would also trigger the extra damage from the debuff your Q left. Yep, even people 2000 range away aren't completely safe(For comparison, Lux's laser is 3000 range). These little gimmicks are the reason why I play LeBlanc. I haven't found a more satisfying way to kill people yet(ok maybe a super gimmickey field of teemo shrooms is a close contender though.)

[E+R(E)]Chain Two people.The use of this is pretty self explanatory. If a situation calls for two slow/snares rather than burst DPS then you can try to chain two people.(eg, you can't kill runners but your teammates will be there in 2 seconds, or snare the same runner twice to buy time for the calvary to arrive) Otherwise save mimic for a burst combo.

[E -> Q -> R -> W] High DPS combo + Long Silence: If you can nail someone with an ethereal chain and all your cooldowns up, the best combination is to hit them with Sigil of Silence followed up by a mimicked Sigil of Silence, then use Distortion for more damage, after which you can continue to chase, or toggle distortion again to return to safety. This combination is pretty safe because Q+R will keep them silenced for about 4 seconds; the mimicked Q will trigger the first 2 seconds of silence, and the second debuff will be triggered by the second tick of ethereal chains. At this point in the combo it is safe to W in because your target is snared and silenced. To maximize the duration of silence E -> Q -> R should be almost instantaneous (smart casting can help), so the second silence isn't replacing too much of the first silence. Naturally against certain targets this won't matter too much, but if you are going for someone who has a skill that would really ruin your gank (eg, Zilean's ult, Nasus' ult, etc), a full 4 second silence would be preferable. This combo is great because it is really safe, you have 3 seconds at the end of the combo to return to the beacon W left at your discretion. The downside is you have to be able to initiate with ethereal chains which can be difficult if your target are hiding around creeps or using their tanks as meat shields which is often the case.

[Q -> R -> W -> E] High DPS combo + Short Silence: This combo is almost the same as the combo above, except there is a much lower likelihood of getting a 4 second silence unless you want to wait 2 seconds between using R and W. (actually very feasible if you have Rylai's scepter) The advantage that it offers over the combo above is (1) you don't have to start with a skillshot that might be blocked, (2) your target is silenced almost immediately, (3) you can bypass obstacles such as creeps when you distortion in for a guaranteed hit with ethereal chains. The disadvantage over the previous combo is that it is not as safe for you to maximize damage output, because even if you instantly casted ethereal chains after using distortion, the chains would snare after 2 seconds, leaving you only 1 second to retreat with your W-Beacon if needed. In addition if you triggered a Sigil silence with your W, the silence will have worn off for at least a split second before ethereal chains snares the target. Lastly, because you have less than 1 second before your W-beacon disappears, if you do choose to retreat you might not be maximizing the usefulness of the snare duration.
Another possibility with this combo is returning to the W-Beacon immediately after you connect an ethereal chain. Ethereal chain has a range of 950 and Distortion only has a range of 650. That means you can keep the target at a comfortable distance while still hurting them with the second tick of ethereal chains.

[W -> Q -> R-> E -> W-beacon]Bluepiller Assasination: I use this combo almost exclusively to snipe people bluepilling. It works great when you show up to a lane just a little bit late for a gank but the target doesn't know you are around, or thinks he is safe at the tower. Three seconds is plenty of time to execute Q + R + E and also get back to your W-beacon. If you chose to run ignite you can seal the deal even more easily. If you chose to run flash you can even assassinate people who are far behind the tower (600+450 distance away). Use flash AFTER distortion to ensure safety; you will be able to chase over a greater distance and also return further back to safety. Don't forget that returning to your W-Beacon will not stop any tower shots that were mid-air from hitting you.

One of the most important things to keep in mind is don't take hits you don't have to. Don't tank creeps, don't tank towers, and when you have to take damage to put some hurt on the opponent, make sure its worth the HP you lose. Don't get harassed into losing your passive either, because it can make or break a gank.

Farming
As mentioned before, LeBlanc is a really sub par farmer, you will really have to rely on last hits to farm with her. Fortunately her autoattack has a fantastic animation; furthermore, Sigil of Silence is on a short cooldown and not overly mana intensive, so it can be used to farm pretty well. I take mid a lot (especially for certain matchups) and because I only last hit, I'm usually defending at tower. Autoattack and Sigil of Silence are pretty decent for last hitting the melee creeps: auto when you get lucky and there is only a sliver of life, Q when an autoattack wouldn't do it, and autoattack + sigil if that would kill. Another easy trick is to take one shot each at all of the ranged creeps as a wave approaches your tower; when the tower gets around to shooting them, one shot will leave each with a sliver of health for you to snipe with your super autoattack.

Lane Harassment
Because LeBlanc is really fragile and really has no way to restore HP besides pots (and her farm is bad so it is rare to be able to waste money on hp regen), try not to take damage while harassing. This isn't too bad because there are a couple easy ways to harass. Before level 6, you can use almost any combination of her skills to harass safely. Q -> W is an easy one: just throw a sigil of silence in, then pop in with a Distortion to trigger the silence and extra damage, then back to your W-beacon. Usually I can get away with this and only suffer damage from one round of ranged creep autoattack that were still midair when I returned to the W-beacon. If there are easy shots to land an ethereal chain, E -> Q is effective particularly if the target is chained for the full duration. This will most likely not only get you the extra damage from the silence debuff, but also leave them in place for a couple autoattacks. You always have to keep ranges in mind though; against enemies that have really killer moves that match the range of Sigil of Silence (700 range), you have to be really careful of what kind of retaliation you're looking at if you want to trigger the Sigil's extra damage with a Distortion. Some players have the twitch reflex and/or experience to anticipate this kind of harassment and you can possibly eat an ugly CC when you go in.

Pick the enemy without regen to harass. Since the nerf to dorans, enemies using ring or sword are generally nice targets for harassment. If I am soloing and my opponent has regen (ie, doran's shield) I generally try to save my Q for farming at first(generally only spending mana to keep myself not topped off.) Dealing damage with low level skills, with plenty of time for the enemy to regen the hp back is kind of a waste of mana. I hurt them a lot more betweens level 4 and 6 just by virtue of Q being so much stronger. The damage I do before hitting 6 is important preparation for turning level 6 because once you have mimic the enemy should be afraid. If I did manage to harass well, my opponent is kind of trained to expect a certain amount of damage from the Q + E or Q + W combination. Once you hit level 6 you can catch someone off guard with the extra mimicked Q's damage. More experienced players will be aware of this but may not be taking into account the extra damage a mimicked skill does(sadly it's only +10% at level 1 though). Against certain characters it will be really easy to keep your HP high while harassing in preparation for a showdown at level6. LeBlanc's burst is nothing that special before hitting level 6, so usually I only strive for safely harassing the lane opponent to a level where I can actually burst them down at 6.
The same principles generally apply when you have a good lane partner. Lane partners with CC are great because it allows your glass cannon powers to really shine.

In team fights I usually look for an enemy DPS who is really soft to pick off after CCs have been blown and I can do so safely. Any combo involving a mimicked Q and all of your spells hitting a soft target should usually leave them dead or dying. Once all cooldowns are blown, LeBlanc is not that useful until Q is back up, but even then the damage from a Q alone is far from stellar. With a bit of CDR you should be able to have your CDs up pretty frequently. If you are forced out of a fight, try to hang out around the periphery (pay attention to what the enemies are up to you can do so safely). When you have cooldowns again it is very possible to still contribute to the team fight. 950 range on ethereal chain means you can help teammates kite from relative safety. If there are walls or bushes to hide yourself around you can wait for a good opportunity to Q -> W -> E -> W (assuming you blew mimic earlier. If mimic is up, then one of the standard combos) safely. Because I play with soulstealer a lot, stick around fights if its safe just to snag some easy assists with ethereal chain or distortion.

LeBlanc also has the ability to deal good damage from a distance too so its rare for you to ever have to be less than 700 distance from the fray (excluding your momentary presence if you need to use Distortion to get in some damage). If the enemy initiates on your team's DPS sometimes it can be nice to use ethereal chains to help peel for your team mate.

Basically, be smart about when you go in. Don't be a hero (thats the tank's job? haha) Just focus on your job of wtfpwning someone that you actually have a realistic shot of killing. (ie, one of the worst things you can do is unload all your cooldowns into 250 MR mundo who has his ult going and then have nothing left for the enemy thats doing all the real damage)

I can't really say that there is one best combination of items because it really depends on the situation, but the following is a list of items I commonly use in a variety of situations. Generally I like to play as glass-cannoney as possible, but sometimes thats impossible so there are a couple survival items I regularly turn to. The principle behind what items I select is generally a balancing AP versus survivability depending on the needs of the game. It is pretty easy to stay out of trouble with LeBlanc because of the range you can be effective at so I don't really get things that allow me to be tanky by any means, but provide some safeguard against threats that can't be avoided.



Starting Items: Dorans ring is a solid starting item, even without an HP pot if you are confident that you can capitalize off the extra life and AP. If I am going a side lane I will often get meki pendant and two HP pots because I can stay in the lane much longer.

Soulstealer: This gets its own category because I almost always get Soulstealer. LeBlanc has great escape tools, can build get fed this way easily by picking low HP people off, and even still collect assists without too much risk even when she's low HP. This is another reason why I usually choose flash over ignite: because dying with soulstealer really sucks. Sometimes I get this as my second item. If survivability looks like it will be an issue I will delay getting this until getting some HP items.

Boots: I usually get Sorc boots, but if there are a lot of CC threats I get Mercury Treads.

Glass cannon items:

[b]Rabadon's deathcap[/b]: Pretty straight forward, Lots of AP. Enhances AP you already have. Great with Soulstealer.

Archangel's Staff: Lots of mana regen if you aren't suited to carry blue buff or can't get it. Lots of AP too, especially if you built up lots of mana from having tear of goddess.

Void Staff: If your targets are stacking MR then grab this. I usually don't pick this up because I target enemy DPS, most of which cannot afford to be stacking MR at the expense of damage. LeBlanc can assassinate enemies when their tanks are preoccupied so void staff is usually not necessary. When deciding when to buy this, don't forget to consider that Void Staff is more cost efficient the less flat magic pen you have.

Survivability items:

Haunting Guise: Easy choice for early/mid game survivability. Cheap to assemble, and magic pen is always welcome too. Even if you have a Soulstealer already you can think of this item as a cheap way to protect that investment while also boosting your damage.

Rod of Ages: Great if you are able to grab it early on because of its extreme cost efficiency if you let it charge long enough. This usually only happens if you miraculously farmed like a champ, or if you get fed off early kills. If you got fed off early kills then it is often prudent to just get a Soulstealer and keep that snowball rolling. I don't find the extra mana too helpful, but can't really complain about the AP and HP.

Banshee's Veil: Obvious defensive choice. LeBlanc is really soft, and this can stop CC that would have otherwise been certain death. Because you don't have to be on the front line to be doing damage it should be easy not to let the shield be wasted on random AOE or other nonsense. Catalyst the Protector is also great for early survivability and longevity in the lane.

Rylai's Scepter: A decent choice because of the added survivability and a pretty useful effect. I usually don't get this because its expensive and because slows tend to work better with chases. With LeBlanc I prefer more AP so I can wrap up business real quick. However if you are not getting AP at a pace fast enough to almost instagib enemy DPS then it may be a good decision to try to support your team more with slows. If this is the case then items with CDR are good choices to help you achieve a more support type role as well.

Abyssal Scepter: Great item, I get this frequently because a lot of collateral damage in team fights tends to be magic, and of course the MR reduction aura is great for your team.

Zhonya's Hourglass: Great item if you need armor. If a mellee is on you and you have ethereal chain up you can nail them with an ethereal chain and then activate stasis. After 2 seconds they typically will either(1) have run range of the chain range(ie, 950+ distance away), or (2) be somewhere within 950 distance, but be snared. Either way both scenarios are better than Master Yi standing on top of your golden body waiting to resume his hacking and slashing.

Etc:
DeathFire grasp is a pretty interesting choice on LeBlanc. Most of the time if I have a ton of AP then getting this item isn't really necessary (again this has a lot to do with targeting squishy DPS enemies). Naturally it really shines if there is a target that you can just barely kill and the extra nuke makes the difference. It can actually make big difference actually on a full HP target. With 300 AP, DFG will do 40% of target's current HP (subject to MR of course, but you got some Mpen right?) Usually though I feel like I am overpaying for this item. To be fair though, I haven't actually tried it that many times because I have had little incentive to deviate from my standard straight AP builds.

Don't forget to top off with Elixir of Brilliance if you have some spare cash too! The extra AP and CDR are invaluable and the Elixir can easily pay for itself through kills.