You guys are being mean about not telling him the secret Kass builds.
There is a hidden Easter Egg for Kass. Once the game starts, you must not move, buy anything, type anything until the passive exp. gain brings you to level six. Yes, your team will probably say stuff like "Report AFK kass." Don't listen to them, you will show them how powerful Kass can really be.
Once you hit level six, only put a point in your ult, nothing else. You have to safely travel to the other side of the map, in front of your enemy's Nexus. You have to use your ult to go across the Nexus to the enemy's summoner platform. This causes the hidden Easter Egg to activate.
Kassadin can now buy the item "The Eye of Sauron." This is probably the most OP item in the game and it is game changing.
The Eye of Sauron
Map: Dominion (Hidden)
Stats: +150 AP, +500 Mana, +150 MR, +50 Mana Regen per 5 sec
Passive: Unique: +30% Ability Power, Gain ability power equal to 5% of your maximum mana.
Active: Unique: Enters into an immobile spirit form for five seconds, which allows continuous casting of spells for duration of the active. 90 second cooldown.
Also, I forgot to mention that you are not allowed to use any masteries or runes with Kass for this to work.
If you do not follow my directions, then you will just die on the summoner platform. Unfortunately, this only works in matchmade games so you can practice in a custom game, but the hidden Easter Egg won't work unless it is a real game.