I dont really have anything against this page, but when I saw other people's Kass build got really angry and decided to post this build myself.
No defense runes? Why?
Why: You can't tank anyone in late game, it's better to prepare yourself to deal tons of damage
No mana stack? Why?
Why: Its nearly impossible to land a x7 stacked riftwalk in a teamfight without dying in less than 2 secs
No magic penetration?! WTF IS THIS
Why: Come on. Don't be stupid. If someone is getting magic resistance items there is no way you can do anything by getting a damn Void staff early (You can only buy one after you have more than 1 needlessly rod. Otherwise, there is no meaning in magic penetration if you dont have tons of AP)
OMFG NOOB! NO HAVOC?!
Why: Its a waste of mastery points. To get havoc you have to spend points on lots of useless masteries. I'm the living proof that you don't need havoc to get 20 kills without dying at all
How to play kass:
Level 1-4: Go mid lane, alone. At this level dont try to do anything risky. Even if you have 20 of regen per 5 sec, just use it to last hit creeps.
Level 5: Step one: Since you went mid, you will usually face squishy heros, so by using Null Sphere and getting their HP to 500(NOT LESS, NOT MORE) you can kill them easily when you get to level 6.
Level 6: Step two: If you didn't succeed in trying to lower your opponent to 500 of hp. Do it ASAP. Then when they have let their guard down. Riftwalk, nullsphere, force pulse, attack melee and ignite if you think they escape. It is a sure kill
Level 6-8: Once you killed your opponent, try to last hit as much as you can. You will have like 1100 gold. Then you have 2 options: 1. gank a lane to get a kill 2. recall and return to your lane again to kill your lane oppponent again (or until you have 1600 gold)
Level 9-11: If you did what this annoying build said until now, you should have 1600 of gold. Buy a needlessly large rod, then gank like crazy, you should be able to kill ANYONE with 110 points of ability power (if you pull it out quick)
Level 11-18: Go for zhonya, buy a brillance potion and resume ganking, you should be able to kill anyone except tanks. Remember, kill squishy heros as much as you can, you can even kill them in a nullsphere+force pulse combo without letting them do anything.
Item Build (optional): You can go for a lich bane if you want to (though I don't use it that much)
Keep these things in mind while you play:
"You can deal lots of damage if you want to."
"You can kill anyone if you want to."
"Don't take fights when they outnumber you"
"You're not a tank, you're a mage"
I main kassadin and I have tested lots of builds. I can reasurre you that there are no better kassadin builds than this one.