Hello! This guide is to help players who want to try and use Kassadin on The Twisted Treeline. If you have anything that you want to improve on in my guide, let me know and I may or may not consider it. I play Kassadin on normal/ranked, and I also play Gangplank (but that's for another time.) Have fun with this build! :)
By the way, if you want to add me on League, my name is Devèstation.

+Pros+
+Very bursty assassin mage. Counter to almost all other mages in the game.
+Can jump over ALL walls in the 3v3 map, if done right.
+Has a spell that restores mana per hit.
+Scales extremely well with AP.
+Is extremely mobile.
+Longest silence in the game (2.6 seconds at max rank)
+Takes 15% BASE reduced damage from magic damage.
+Has a naturally high attack speed. (~1.039 at max level)
+Riftwalk Does 60/70/80 extra damage each time it's used within a 7 second period.

-Cons-
-Squishy in the beginning.
-Item Dependent
-Riftwalk takes 100 extra mana each time it's used within a 7 second period.
-Can only focus 1 person down, effectively, at a time.

Farming is essential in games that they have 2-3 good tanks in the game. You have to farm to get the high-end items so you can start destroying their team. It's easier to farm under turret as Kass, but if you're either lucky or playing right, you should have them at at least midlane almost all the time. Ganking is great on Kassadin, since he can Port into the bushes in top and bottom lane from the relic platform. IT IS IMPERATIVE THAT YOU GANK FROM THE BUSHES WHENEVER POSSIBLE If you don't, they have a fraction of a second to react, which could mean the difference between a kill and a death/deny

With Kassadin, it is important to have AT LEAST ONE TANK on your team. You can not initiate fights against 2-3 people, even at full build. You have to stay back until attention is on your tank/bruiser. You have to coordinate with people on your team to get maximum damage out of yourself. If you die in a team fight, it is most likely lost. Be extremely careful while playing Kassadin!

Kassadin has a few unique skills that you can use to your advantage. One is that he has a base resistance to magic. That means laning against a Vlad, Lux, Teemo, ETC. should be a piece of cake.

His other one is his extreme mobility on the TT. He can ~Literally~ jump over EVERY wall on TT. Try it!