EPTHe's a poking initiating nuking snarebot, that can also get anywhere he wants to ward (pretty safe warding) and can push lanes/towers like a boss. There's almost nothing you can't do with Kassadin, so this is entirely your playstyle. I play him as a tanky initiating poking snarebot, generally de-prioritizing actual damage output (hence not getting ZRing as fast as people like GJ) - most people don't, they play him like this whonky terrible-burst (only two nukes) nuking vulture but hey, whatever floats your boat as long as it works.
Different playstyles include my Tankassadin, Grandjudge's Vulture Kassadin, DOGkaiser's DPS Kassadin, and statikk's SS/GA/Clarity Kassadin, among many, many others others (Eu DFG Kassadin is also a playstyle, although I think it's a horrible one and only works against bad players/team comps). Especially true when you consider that Kassadin is a highly flexible hero with a strong base selection of spells and a relatively high ceiling for potential (although I still maintain ranged DPS > him, especially teams that babysit their ranged DPS). Knock yourself out, the "line of thought" posted first below (I'll add other stuff as people post them) is my tankassadin and is (as mentioned) pretty different from most players.
Also, sorry for most of this being copy paste mish-mash from the doomed thread and the guide I wrote months ago. Nothing has changed since I wrote my line-of-thought guide (except for W getting shorter cooldown lol) so it's still 100% applicable.
Masteries
0/9/21 non-negotiable. He needs the summoner spells (esp. Cleanse), he needs buff duration, he needs meditation, and he needs SoS.
Runes
You have flexibility here based on your preference. I run MPen Quints and Marks, mana regen per level Seals, and AP per level Glyphs, with one MPen Glyph to let me hit 15 MPen. Some people like having Flat AP Quints instead.
Skill order and play
This is gonna be long and more of an essay than a clear-and-cut guide, and maybe someone (like Neo) will pop it and change my mind on a few things etc. but this is the sort of "flowchart" I follow when I play Kassadin.
Ideally you want a solo lane, IMO, because he's a very dominating solo hero (he plays a bit like Pantheon) and you really want the level advantage to hit 9 ASAP, 9-13 is like the sweet spot for Kassadin because your nukes begin to hit their max firepower, which is important because he only really has two.
Soloing is relatively simple and yet not. For the most part your primary spell is going to be Q, due to its relatively low cost for its damage. Beyond that, a lot depends on who you're facing.
Against most physical DPS you're going to do something like Q W Q E Q, although in some rare cases you may go Q W Q W Q instead (I did it against Kayle) - I think I ended up doing double W because I didn't expect anyone to come (it's annoying ganking Kayle) and I wanted to overwhelm the Kayle with Q spam to supercede her heal - that's where the mana came in. And if she ever tried to stand and fight the higher armor pen let me actually outdamage her. Against Sivir I do something like Q W Q E, I want that early W to pop off the shield and actually land a good Q. One E I found was enough to outdamage her in a standing fight. Against Morgana I just massed Q and W for the same reason as Kayle - I wanted to pound down her shield and out-last her mana-wise. However if you're against someone with Clarity and/or Teleport you switch into farming mode, because you can't outlast, etc.
In the rare circumstance you're 1v2 against two melee, or are 1v1 against a melee, you can raise E over Q, as E scales better (kind of like Annie's W > Q), and E is a much more important spell in teamfights anyways. Even if I open with a stronger Q most of the time I stop at like level 3 Q and just pump E to bring it to 5 ASAP, as it's crucial for teamfights, given that it cools down almost twice as fast and is AoE.
In a 2v2 you're almost always going to level E over Q, and for the most part you ignore W altogether (as is the case against two melee usually) because you're not gonna need that mana return as desperately. It kinda depends on the lane, though, sometimes you'll find it beneficial to grab an early W for extra mana help and to smack people back if they try standing up to you. Always remember that grabbing a level of W is a weaker Q or E, and by relation ~30 less damage every harassment cycle, which adds up really fast. Only get it if you're absolutely certain you just want to whack them down via attrition, because W is largely useless in teamfights. It's hard for me to tell you how exactly I know when to get W, because I usually do it by instinct. Just try to look at your mana expenditure, damage output, lasting power, etc. and try to plot out what you need. It's not super complex but if you mess up it can slow you down a lot.
I favor Cleanse/Ghost. Because of the way you position yourself, and who you tend to go after, you end up being public enemy #1 for a significant amount of time. Between an Annie standing in the back and a Kassadin teleporting into your team to fuck up the carry, who do you think is the easier, and more urgent target? Yeah, the Kassadin. If you play REALLY careful you can get away with not having Cleanse against minimal CC, but I've found I get into the habit of relying on it, and die to the only CC on their team (Rammus taunt) etc.
A lot of people like Clarity, but I think if you're in a solo lane W can trump Clarity, especially if you land a few champion hits, so it's much more of a 2v2 domination spell. I have trouble using it, though, because I'm really stingy with spellcasting, so that's going to be up to player comfort with Clarity. It's nice to have the odd time that you jump too much and need one more bit of juice to escape etc.
I've not taken Ignite because I feel like for the most part Kassadin himself is like an Ignite - if there's a weak target he can usually get them anyways, especially if you take Cleanse.
Flash is nice, but it's about distance covered for me - Ghost will save you in places Flash can't, and vice versa. I'm more comfortable with Ghost, and have randomly died because I didn't realize Flash wouldn't save me in a situation Ghost would, etc.
I run 0/9(SoS)/21 Masteries, reason being that I love Meditate, faster cleanse (from uti21), and the extended neutral buff duration. Because of his nature as a spellcast harasser creep hits usually aren't that big of a deal, anyways. 15% magic pen is overrated IMO, the 3% cooldown is nice but not really worth the 9/0, so get the 6 armor, MR, and some decent hp regen. Better than nothing, anyways.
Item builds, wow I go all over the place here. The only real constants are that I open Doran's Shield + health pot, and I make Catalyst and Sorc Boots, unless it's late and I find I'm just being CC'd to death (then I'll switch to Merc, especially once I've finished Void). In a solo lane you have, if played well or simply better than the bad opponent, been able to farm well due to your OP Q (mm, tears). After Cata, it's really up to whatever you want to do, ala the playstyles I listed way at the beginning of the game, or whatever else you want. Cata's EHP is simply ridiculous, and its level-up effect basically wins you lanes. I have seen some people double-Cata because he's not as reliant on AP, and I've also seem some very good Kass players open Banshee first. Especially if there's a mirror-match - although this worry is obviously not for ranked games though.
I used to rush Soulstealer, but it's risky business. My typical Kass build sidelane Soulstealer into Guardian Angel into Void Staff, but depending on the game I've ended up with weird stuff like 4x Ring into Tear into Abyssal. He's very flexible, and doesn't need much money to dominate the time period he dominates (~level 9 to level 14), and after that it's just a matter of staving off father time via gold. Even with 20 stacks you'll find you just can't keep up late-game, so you're just a tanky support hero. You really, really want to end games in under 35 with Kassadin if you want to remain a relevant nuking carry.
Items I've been known to use:
Tear/Arch
NLR/Zhon
Soulstealer
Leviathan
GA
Abyssal Scepter (for MR)
Void Staff
Heart of Gold
Aegis of the Legion
Sheen (for mana)/Lichbane (mana + MR + proc)
Haunting Guise
Rylai
Positionally he's got some bizarre issues of having one of the most accessible escape/chase spells (that's expensive), a really long-range poke/silence, and an absurdly short-range aoe/poke/snare. What you'll find is that you have to have a really good mental image of the effective ranges of different heroes, and you also find that you chase/engage on foot (instead of teleporting) a lot, unless you're trying to surprise people or needing to cover ground in a hurry. People, in turn, will try to stay a whole screen away from you to force you to teleport for initiation - this lets them get a countermove on you, where you teleport, snare, but they just jump at you, and you've got seconds until your teleport out. His most effective spell is Force Pulse, it's your bread and butter, and it's really tough to use. If you can get good at spacing for Pulse, and can position yourself to constant catch multiple enemies (bonus points if they're squishies) during teamfights, you're on your way to abusing one of the nastiest spells in the game. On the other hand if you're getting ripped apart trying to space it, or die misplacing yourself, etc. then you'll feel pretty weak.
One of the tricks I use a lot initiating is to teleport in just into range, pulse, and ghost backwards, or I'll ghost up, pulse, and teleport a half-step away so that if they're still running I'll have a charged pulse and be able to teleport up to catch them again. If they're running Kassadin has a field day because he can port, pulse, and smack them with autoattack. And in teamfight skirmishes as you're pushing a tower, you're almost always at the front, poking with Q, harassing with E whenever you get the chance, and just being a general nuisance maximizing your damage output
For everything else, check out the spellcaster thread I'm working on :D /end shameless self plug for something that's not even REMOTELY close to done. But that's okay.
Dinky section on abusing Q
Because I love having the freedom of an OP where I can add sections at will.
It occurred to me a lot of people, both for and against Kassadin, don't realize the most douchey thing Kassadin does that wins lanes if you're on top of your game. Abusing the fact that he has like a 0.1 cast time, if you're dancing, then one thing you can do is time your "turn" just at outside reach; this leads you to poke your sword out and out flies pew pew lasersphere of OP-ness. But if you've clicked back, he instantly turns around and runs with hardly a hitch - this means the other player had to have responded FASTER than you. The toughest part about this is timing your in-and-out so that your Q radius barely outreaches their range, but if you're on top of your game anyone with less range cannot hit you.
Takes skill, and I myself am inconsistent at it (I have good days and bad days). But if you really want to master Kassadin this is your #1 priority (#2 being spacing E, #3 being last-hitting).
Afterward
Sorry if this doesn't seem very absolute a guide. I hate stamping my foot down and saying PLAY THIS WAY, because that leads to flow-charty drones that suck at the first sign of strange-ness (and yes strange things happen in this game). I'll encourage people to do something, state my reasons, and let them decide, or discourage something, etc. but I really hate going THIS IS WHAT YOU DO, especially on a flexible hero like Kassadin where evidently going DPS works okay for some people like DOGKaiser, bless his heart. I can't fathom doing it. I just don't play in a structured enough way to write a guide that's much more than a patchwork of "things I think about" as I buy items/level-up skills/etc. over the course of a game.
Hopefully the OP gives you a good idea of how to approach him, though, and the pros and cons of different items and skill builds, and lets you think about how you can take advantage of him the best you can. And if you have any questions we're here to help :D
TL;DR version
Cleanse/Ghost
0/9/21 for SoS/Medi
Q:Mpen
M:Mpen
S:Mreg/lev
G:AP/lev(+1mpen)
QWQEQR
R>Q>E>W
Shield -> Cata -> w/e you want
