Alright, this is my first guide, so don't hate on me! :)

Since day one of playing League of Legends, I have always looked at Kassadin, thinking, "Hm...he seems kinda dumb. Maybe I'll try him some day though." I pushed off buying him for as long as possible, but it all changed when I saw a Kass go 25/1/2. I immeadiatly bought Kassadin, and played a few custom games with him. I failed. Soon after, I discovered Mobafire...and found "Kassadin: Badass rift ninja". Best. Guide. Ever. It got me into scores of around 10/2/6...and after tweaking the guide, and adding some Oozlaka to it, I'm now getting games of 20+/3-/5-. So, look at the guide...get familiar with it...try it...then leave comments! Hope you guys enjoy it!

There are many things I like about Kass, but in all fairness, I'll add the things I don't like as well.

Pros

~ Fantastic harasser.
~ Long lasting Silence.
~ Amazing escape/chasing abilities.

Cons

~ Extremely squishy early game.
~ Often targetted in team fights.
~ Tough to get used to playing.

There are a lot more of both Pros and Cons, but I'll let you guys figure them out!

I follow the Badass Rift Ninja guide's runes; Greater Marks of Insight - for early game damage on squishies - Greater Seals of Clarity, and either Greater Glyphs of Focus, or Greater Glyphs of Force. For Quintessences I take Greater Quintessences of Fortitude, for extra health.
The combination of these runes allows for massive damage to get fed, while wasting little mana due to the regeneration.

I take standard caster masteries - 9, 0, 21, making sure to get the extra 15% Magic Penetration in Offense, and Cooldown Reduction in Utility.

Once again, these masteries help with damage output (especially early game), and reduction.

A lot of guides, and people, suggest taking Cleanse, and some other spell. And while I'm not gonna say DON'T take Cleanse, I will say it isn't my favorite choice.

I prefer to go with Ignite and Clarity. This two SS give you durability (mana-wise) and allow you to take that last 60 health your enemy gets away with 1/2 the time. Using Ignite and Clarity effectivly will be explained later.

Other choices you may try include: Cleanse, Flash, Ghost, and, believe it or not, Clairvoyance (used to determine whether or not the champ with 60 health left is still at their turret or not...). Sadly enough, I've also seen Kassadins use Heal, which...just isn't the right thing to go with, in my opinion.

Try to max out Void Sphere, followed by Force Pulse and Riftwalk, saving Nether Blade for last...the order I take is - Q,E,Q,E,Q,R,Q,E,Q,E,R,E,W,W,W,R,W,W. A lot of Kassadins take Nether Blade early game, but I've never seen one who does this play successfully.

Void Sphere is one of your main harassing abilities, and should be used often, to keep the enemy champ at half health or a little lower, so that when you get Riftwalk, you can charge in and get the kill (which is often first blood). I will go into depth with harassing a bit later.

Remember, this guide is explaining how I play Kassadin as an AP nuke. NOT Tankadin (Which I personally think is the most fail way to play him. Any champion at that. Ever.) So, on that note, here's how I build Kass as a nuker.

I start out with a Saphire Crystal, and a Health Potion and Mana Potion, for early game mana and durability. After that, I build my Sorcerer Boots, followed by Mejai's Soulstealer, and then a Void Staff. However, as The Angry Cookie mentioned in the comments, the Void Staff is a very situational item. Although I typically take it anyway, you can skip the Void Staff if the enemy isn't stacking Magic Resistance. Afterwards I'll buy Rabadon's Deathcap, and by this time, most games are over. If, however, the game is still going strong, continue into an Archangel's Staff, as you should still have that Saphire Crystal from earlier. For your last item, buy either a Deathfire Grasp, Lichbane, Zhonya's Hourglass, or an Abyssal Scepter.

Make sure to let your team now you're stacking with Mejai's and ask if you can take most kills....majority of the teams out there will be fine with a carry, and gladly accept this. Also, if you're playing with a smart team, you may notice them sacraficing themselves to save you once you have a good amount of stacks. That always feels good! Be sure to thank them if they do!

Personally, I'd much prefer going mid as Kassadin, but I'd suggest laning before going mid if you aren't comfortable with Kass just yet.

You'll still want to take the same items, runes, masteries, summoner spells, and ability order, but gameplay will be a bit different. Make sure you alert your laning partner to who you want target and take down first. If the enemy target's laning partner isn't too close, go ahead and throw some Void Spheres every now and then, just be careful not to get slowed, snared, etc.

Once you get your ult, much unlike midding, you'll want to be careful with when/how you use it. You can't just jump in with it, chances are you'll get a CC of some kind, and almost, or will be, killed. Always use it as an escape mechanism if need be, but other than that, leave it alone for now. When you and your partner have the target at low health, ping him, and give a warning that you're ready to go for it. Once your partner agrees, Riftwalk in and unleash your Void Sphere and Force Pulse, then ignite him, and melee. With your partner, this combo should easily kill one. Afterwards, if you aren't too low on health, go ahead and do some damage to his partner, otherwise, Riftwalk back out. Keep this up, and you should build up some kills in no time!

Ah, my favorite part of every game. Taking mid, and not failing at it.

As Kass, you'll definately want to call mid in the champ selection screen, but, if someone else calls it first, let them have it. You're playing a good laning champion as well.

I haven't seen a single midder I can't easily beat as Kassadin using my build. Malzahar is the ONLY, I repeat, ONLY, champion I can't harass early game. Mind you, this changes at lvl 6, but if you ARE by chance laning against a Malzahar, be careful, and only fire of Void Sphere when he comes in range. Avoid being hit by Malrific Dreams, they'll tear you down fast if he uses it repeatedly.

Try to avoid using your potions untill below a comfortable health and mana lvl...for me that would be about 1/4 health. If there's an MIA, about 3/4...speaking of which, MAKE SURE YOUR TEAM CALLS MIAs!!!!!!! I've died many a time due to an uncalled MIA. Now, if it happens, please don't rage, just suggest a friendly, "Call MIA's please!", and if it happens again, point it out again.

Now, back to midding. Assuming you aren't against Malzahar, you'll be able to throw in Void Sphere pretty often, but don't get too cocky. Remeber, keep your opponent down to, or below 1/2 half for your lvl 6 combo which is coming up soon. Keep harassing, and try to avoid getting too low on mana. If you do, use your mana potion, but try to avoid health potting till lvl 4-6.

Congradulations! You've hit level 6! And guess what that means? You win a free early game kill! Now, go ahead, do it. C'mon! Um....hello? Oh, that's right, here's the combo - Riftwalk, Void Sphere, Force Pulse, Ignite, spam your auto attack, and as soon as it's CD is off, either land ON TOP OF (Very important) the target for that last bit of damage then run out, or, if he's at his turret, port out, using Riftwalk.

Time for the all important skill any LoL player should know - How to Properly Gank.

As Kassadin, try to be at least 8-9, so you have your most damaging ability maxed out, and you have your porting ability. After killing your current opponent, drop out of the lane, going either to top, bottom, or, as The Angry Cookie mentioned, middle. However, I wouldn't suggest ganking mid if the enemy midder is somewhat near their turret. Otherwise - go for the middle gank! Anyway - hide in the bushes below the top lane, or above the bottom lane, and tell your team to let them push that lane. As soon as they're a little over half way, ping your target, and start to run in (not Riftwalk in). Once the enemy starts to make a run for it, port ahead of him, using Void Sphere followed by Force Pulse for that devastating Silence/Slow combo. This will allow your team to catch up, in the mean time, Ignite the target, if you have it available, and auto attack it, using Riftwalk as often as possible during the fight. Once the target is dead (I certainly hope you can land a gank kill...), go ahead either back to base for another item, or head back up/down to middle to continure your conquest.

Rinse. Wash. Repeat.

Remember, ganking is your greatest ally while playing Kass, and most other champions.

I hope you enjoy my Kassadin guide, and use the fact that a lot of people don't know about Kass to your advantage. Updates will come shortly to fix any mistakes, and add more (Ranked Play, Farming, etc.)

Comment below, I love feedback (Whether it's good, or bad!), and above all else, Have Fun, it's just a game after all~

Oozlaka