This build came into being as an odd progression of my dissatisfaction with late game Leblanc and my love of tanky characters. I picked up Leblanc after getting a little bit bored with Nasus, but with Nasus being my main for so long, I retained a fascination with tanky characters. Having said that, my experience with Leblanc started off MUCH differently. Extremely squishy, and with her escape abilities on a relatively long cool-down, it required a different playstyle. That was fine, and I adapted to it. Then I found Leblanc's ugly, glaring weakness: By late game, it's extremely hard to keep Leblanc's damage output up. Her base damage is quite high, but her scaling just can't compete with other nukers, a lot of which have more reliable CC than her less-than-ideal root and her admittedly spammable silence. Thus, I decided to take a step back to my experiences maining tanky DPS characters. The relation here is that by late game, Leblanc's damage output is more on par with a tanky DPS than a pure nuker, so I decided to give her the advantage that tanky DPS have: Durability without sacrificing much damage. The objective of this build is to provide a medium to successfully take advantage of Leblanc's ridiculous early game burst to provide enough momentum to secure your usefulness as a late game tanky mage who does decent damage while being ridiculously hard to pin down and kill. I've used this build many times successfully and I hope it will serve those who choose to try it well.

What we're going for here is tanky mage, so I chose armor penetration reds to give you that early game oomph to help you snowball, and and the flat AP early on gives you a pretty good boost to your poke early game. The armor yellows and the HP quints are there for the sole purpose of surviving the poke war in your lane long enough to either kill your opponent or reach level six and kill your opponent anyway, which I've found is almost always the case if you're consistently harassing.

For the masteries I chose 0/21/9 for extra tankiness. Veteran's Scars adds a beautiful HP bonus and the 4% damage reduction is pretty superb, especially with the bonus from a fully stacked Leviathan. I can't get enough of Awareness either, and with Leblanc's early game zoning capabilities, it shines even more. Other than that, the only thing of note is the enhancements to teleport and clarity.

First item should always be Doran's ring. Extra damage and mana regen, yeah, but more importantly, between runes, masteries, and the ring, you'll have a solid seven hundred starting HP, and if you're taking the mid lane as you should be, you'll probably be against someone who is quite squishy, meaning that you can survive their poke a lot better than they can survive yours. The second ring emphasizes this difference even more, especially if you get first blood and can buy it early. The earlier you get the second ring, the better, unless you can straight buy Sorcerer's Shoes, which come either when you can afford to completely buy them, or after the second Doran's Ring, whichever comes first. The build branches off a little here. If you have three or more kills by now, go ahead and skip the Haunting Guise until you've built Mejai's Soulstealer and Leviathan. Now, some people might give you funny looks for building Leviathan on Leblanc, but trust me, this build will almost certainly NOT work without it. I'm serious. Leviathan is the key item that brings the build together. As strange as it might seem, it's what enables you to snowball effectively with Mejai's. In the early to mid game, most people don't have the pure damage output to deal with high HP unless they rush Madred's Bloodrazor. Adding to that, there's a good chance people will still try to deal with you the way they deal with the average squishy nuker, and that will enable you to cause enough chaos to at least halfway stack both snowball items. After you get those items, everything else is really icing on the cake. Rylai's Crystal Scepter will make you even harder to kill and harder to escape, Rabadon's Deathcap provides a good damage boost to casters with even the worst AP ratios. Once you've gotten that far(you'll have sold your Doran's Rings by now), you can choose to trade your Haunting Guise in for a Void Staff. The magic penetration is good, and by now you'll have enough HP not to need the Haunting Guise. Once your build is complete and you are snowballed, you'll be nigh unkillable.

Sigil of Silence: Bread and butter damage skill. Short cooldown, not a bad mana cost, and added silence and bonus damage if you hit with another ability shortly afterward.

Distortion: Oh my, what a cooldown. Twenty seconds at level one will seem agonizing, but in the early game this will be what you use to augment your harass by detonating your Sigil, or, if they are playing very defensively, it can allow you pop in range, Sigil, and pop out without taking much damage. A very wonderful towerdiving skill. Also, being a dash with a pretty big range, it makes a wonderful chasing and escaping skill.

Ethereal Chains: I love Leblanc, but I hate this skill. It's not useless, but it has an extremely slow moving projectile that is easily dodged. Land it when you can, never rely on it. It's good for escaping though, because the person chasing you usually won't dodge it. You can also use it to detonate any Sigils that you missed with Distortion. When calculating in your head if you can kill someone, always assume that this skill will miss and plan accordingly.

Mimic: Ridiculous. On any other champion it would be blatantly overpowered, and the only thing keeping it from being broken on LeBlanc is her mediocre scaling. Almost always, when I get this skill, I secure a kill shortly thereafter. Up to 40% increased damage on any of your abilities is ridiculous, but you want to always try to use it with your Sigil if you're trying to kill someone. There are certain circumstances that call for you to use Distortion with it, and rarely, Ethereal Chains, but as Mimic gives the bonus damage to both parts of Sigil, you want it to be your mainstay. Don't be afraid to spam this to harass, and late game, you can even use it to farm. It has a stupidly short cooldown. It isn't all that uncommon to use it twice in a teamfight, kinda similar to Lux's ult.

Recommended:

Teleport: I am in love with this spell. Increased mobility and ganking power(ask your teammates to put up bush wards for you to port onto for kills, and as a mid champion it gives you more laning power due to ease of shopping. I'd never ever roll without it.

Clarity: You are technically able to function without it, but it makes it a lot harder to secure early kills. With this, you can afford to spam earlier. Don't be afraid to use this whenever you want. The cooldown isn't terrible for a summoner spell, and it keeps the pressure up on the enemy. It's not uncommon for me to have popped it at level one. You might think that this is a foolish waste of mana, but keeping the pressure up on the enemy early on is crucial for zoning during laning phase. All of your early game mana should be used for harassment.

Other Spells:

Flash: Self-explanatory, really. You get an extra escape, and add a third movement ability. Between Distortion, Mimicked Distortion, and Flash, you should be nearly impossible to pin down. Most of the time though, I find it a little excessive and unneeded.

Exhaust/Ignite: Basically serve the same function on LeBlanc, which is a little extra help securing kills. I don't really think it's all that needed though. Even reducing your enemy down to critical health and forcing a retreat qualifies as a vital win due to them being away from the minions more than you.

Cleanse: You're not a tank, no matter how many Leviathan stacks you get. Can give you that extra little push you need if you have trouble escaping.
Spells I would not take under any circumstance:

Clairvoyance: This is better put in the hands of a support. The other spells just add too much potency to pass up.

Revive/Rally: If you feel you need these, something is seriously amiss.

Smite: Jungle Leblanc is fragile, a bit luck based, and very slow. Not worth it imo.

Heal: You just don't need it.

Fortify: Once more, put it with the support or tank.

This is the most important part of Leblanc's game. This is where you sink or swim. Right from the very start, you should be playing extremely aggressively. Note, LeBlanc is a terrible farmer. Last hit where you can, but every time you can, from level one and up, send a Sigil to your opponent. Don't be upset if your last hits suffer a little as long as you make sure your opponent suffers more. You will have superior starting damage output over almost every other champion in the game, if not the best. When you hit level two, use Distortion to proc Sigil's triggering effect. Even if the enemy has health potions, as long as you don't take too many minion hits, you should win the poke war due to defensive masteries and runes. Before long you should force your opponent to blue pill or to tower hug. As long as you keep up the pressure, they'll probably end up staying like this while they are in the same lane as you.

Note, this requires playing more aggressively than is safe for most champions, and even Leblanc requires a bit of caution to succeed with. Beware especially of junglers. They will ruin your day completely, especially Udyr. Shaco is probably the least threat unless you are at critical health, but in any case, try to make it back to your tower if a jungle ganks you. Most smart junglers will wait until you've used your Sigil/Distortion combo before striking, so you'll be without an escape. Fear not; As soon as you and the jungler are clear of enemy minions, Ethereal Chains becomes relatively easy to land. Your entire goal while being chased by anyone is to stall until Distortion comes off of cooldown. It's a risky play, but the pressure you keep on the enemy by Sigil/Distortion harassment is worth it. If you have to play defensively because you are being outlaned, use Distortion to pop in and out of range to land Sigils. It'll be a lot slower, but you should be able to whittle the other person down more. Other situations that can screw you over is if they send a self-healer like Nidalee or a tank like Cho'Gath to lane against you. They won't kill you, but more importantly you can't zone or kill them very well, depriving you of the advantage that you NEED to steamroll late game. Ask for a lane switch if you feel the heat is too much. Don't delay or you'll be near worthless late game.

At this point you'll have hopefully netted a few kills, and even if not, all is not lost yet. As soon as you're able to without sacrificing any towers, start roaming to gank. Ask your teammates to put down wards for you to teleport onto so you can strike at the weak and the squishy with ease. You have a very good chance of attaining at least 10 Mejai and Leviathan stacks this way, which is the minimum of what you need to continue snowballing easily late game. The Leviathan stacks are very important, make sure to back out if you feel they are in too much danger, but don't abandon your team when they need you. If you realize you are going to die no matter what, try to net at least one assist or kill before going down. If you're ganking well, roaming phase should not last long as the enemy will start to group up for safety. Roam as long as you can, but don't be afraid to transition to late game. Roaming too late can get you killed.

Everything you've done before should solidify now. Your kills have netted you stacks and items that make you extremely durable. Abuse this durability to move in and out of fights and do insane amounts of damage with relatively little risk to yourself. It may take a little practice to learn how much abuse you can take, but stick with it and soon you'll frustrate opponents as you make it away after assassinating their carries time and time again. You'll probably end up a very popular target if you pull this off, so use that to your advantage too. Often the enemy will be so focused on you that your teammates can rip people apart while you're being chased. Don't be upset if you don't always get that kill or assist, you're still serving your purpose in creating fear, confusion, and pain in the enemy. In this manner, you can help lead your team to a solid victory.

Teammates and how to use and treat them:

Your team is a valuable tool, especially as an assassin. Don't abuse it though. Before anything else try to maintain a polite tone to your team. You NEED them to be willing to help you. Making them mad at you isn't the way to do that. Congratulate their kills and plays, no matter how much you think they stole them from you, don't rage when they occasionally screw up, and try to maintain a positive attitude. This will make people a lot more willing to help you out when you need it, and help maintain your team's cohesiveness. A lot of the time other teammates will follow the example you set if you act like the nice, mature one.

This being said there's a few ways that your team can benefit you:

Baiting: Low health escape artists make the perfect baits. Little does the enemy know that the moment they chase, you're out of the bush instagibbing someone.

Warding: In addition to valuable line of sight, reminding your team to ward bushes allows you a safe platform to teleport in on unnoticed so that you can add pressure and pain to other lanes.

Ganking: Sometimes all you need is a little push before you attain lane superiority. A well-timed ambush from your team's Alistar can net you that advantage so you can regain or maintain the pressure your lane needs.

Lane holding: If you don't have a jungler available, you can still ask for a lane hold when you don't have teleport up.

You and your teammates can interact in unlimited ways past this, but just keep these valuable tools in mind no matter what champion you are playing.

Self-healers:

As mentioned earlier, they can really screw you up. If it becomes too much, ask for a lane switch if possible.

Standard LeBlanc Combo Attacks:
LeBlanc is heavily combo based. Here are a few of them that I like to use in certain situations.

Sigil/Distortion: Standard laning harassment. Allows you to pop in and out for large amounts of damage.

Sigil/Mimic/Distortion: Becomes your standard harassment once you hit level six. Don't refrain from ulting unless you think you need it to secure a kill in the near future.

Sigil/Mimic/Chain: Only use this if you have a clear, easy shot and you want to save your escape ability.

Sigil/Mimic/Distortion/Chain: Full nuking combo. High damage, and still leaves you open to recalling to your starting position with Distortion. Use it for forcing retreats, or killing people.

Chain/Sigil/Mimic: Damage people during chases.

Chain/Distortion/Mimic: Easy escape from a single pursuer. Feel free to add a Sigil in for the damage.

Distortion/Mimic/Sigil/Chain: Only use on low health targets. Not very much damage compared to the full nuke, but great for chasing one hit kills.

In conclusion, this build for LeBlanc is all about an early lane harassment game transitioning into a tanky, high damage mage. Keep the pressure up early, be vigilant of ganks, and prefer mid lane. Don't be afraid to settle for assists. Stacks are the priority. Play cautiously. Do these, and you should have a powerful champion with a moderately high to extremely high K/D/A, as well as being useful to your team. Good luck and I hope my build helps those who use it.

Feel free to comment, constructive feedback is appreciated.