Hey guys, welcome to my guide for Kassadin! This is my first guide, please don't flame. Constructive criticism is fine, and support would be welcomed.
Anyway, my build for Kassadin is made to annoy people. Kassadin is my favorite champ, though that's not saying much. I am a newbie. I admit it. But that won't stop me.

Pros:

Has a built in flash
Short cooldowns
Powerful silence
Gank-slow
Really great burst combo
Towerdiving!!

Cons:

Squishy
Runs out of mana quite quickly early game
When you die, you lose stacks. Don't die!

Marks are self explanatory.
I like armor seals because Kass needs them. Kassadin is squishy, and has trouble sometimes getting away, if Riftwalk is on CD (which is why you never RW in, unless you're tower diving). Armor seals really help to keep him alive, it seems.
Cooldown glyphs really are nice. Reduces cooldowns quite a bit, letting you RW maybe a second earlier, saving you in some ganking situations, and getting an earlier Null Sphere.
And quintessences are derp.

I like that last point in attack because we are using Ignite as a harassment and as a damaging spell. Ignite has a relatively long cooldown, so you get to utilize that 10 AP, especially early game when everyone is squishy.

Starting Items:
[Boots of Speed]
[Health Potion] x2

We start with Boots and two Health Potion. I don't think you need three, because with Null Sphere and Ignite, you'll be getting your enemies to port or hopefully, killing them before you need them.

Core Items:
[Mejai's Soulstealer]
[Catalyst the Protector]
[Rabadon's Deathcap]
[Sorcerer's Shoes]

Start with a Catalyst, so you don't have to port back or heal so often. Follow it up with your new Sorcerer's Shoes (no brainer, srsly) and Mejai's. After you start getting some kills, bring in a Deathcap for the ultimate ability power pwnage. Without Deathcap, you lose out on a LOT of AP that is desperately needed.

Endgame Items:
[Banshee's Veil]
[Archangel's Staff]

At this point you should be owning or losing, but you should be doing well anyway. A veil can really help you against those annoying casters that like to stand just out of your reach. And by spamming Archangel's Staff, you can really rack up even more AP, especially considering your Deathcap.

I first max Null Sphere, obviously, as it's your main source of attack damage throughout the game. I also max Force Pulse, dumping one point into Nether Blade at 12 to charge Force Pulse and also to kill minions a little quicker. Whenever you can get it, get Riftwalk. You won't regret it. Riftwalk is possibly the best ability flash there is.

If you like, you can skip the Nether Blade at 12 and get Force Pulse instead. It works well, too, but if you aren't doing very well, the Nether Blade helps you get some more income.

Ignite is a really good spell early game, a useful spell mid game, and an overlooked spell late game. Early game, you combo it with Null Sphere and possibly Force Pulse for a first blood. Mid game, you start dishing out some damage, and using ignite against Soraka or Taric to reduce their healing. Ignite can also get that last 50 hp down to 0, and it can't be cleansed off.

Cleanse is my personal preference. You could go Flash too, but you don't exactly need it with Riftwalk on such a low cooldown. With cleanse, you can escape crowd control and GTFO if you need to, and it gets rid of most negative effects, like silences and stuns.

Early game:
Start out mid if possible, but definitely solo lane. Last hit with Null Sphere if you need, rack up the gold. Harass your opponents with Null Sphere and Ignite. After your opponent ports, you should port back too. DO NOT tower dive until you get RIFTWALK, unless you're ABSOLUTELY SURE you can win and come out alive. When you have enough to get your Catalyst, port back, buy it, and hurry back.

Mid game:
By now you should have your boots, and maybe are building your Mejai's. Keep farming, except now, make sure to ward the jungle to save yourself a lot of pain. Knowing when a gank is coming will really help you keep your stacks. Tower dive whenever your enemy is fleeing with less than 1/3 hp, and RW in, blast them with a Null Sphere and a Force Pulse, and GTFO. If you die, don't panic! Be calm and buy your items when you need to.

Late game:
By now you should be done with your Deathcap and owning. Use the combo Q-E-F-R against any fleeing opponent, or Null Sphere, Force Pulse, Ignite, and Riftwalk. In teamfights, use your Force Pulse to hit as many as you can, and pick em off with Null Sphere. Target the carrys and let your teammates deal with the tanks. If you ever drop below 1/4 hp, retreat and port back. Start stacking those Archangel's Staffs, selling what you don't need for space, and remember your Veil. Flee ganks as fast as you can, preferably towards a waiting team to counter-gank them.

I hope you guys liked my guide, and maybe some of you will use it! Anyway, I wish you good luck harassing the hell outta people with Kassa here, and remember to not die and to kill people.

Don't forget the +1! :P

(Remember, no flaming pl0x. This is my first guide.)