Hello and welcome to my first build for kassadin Hope you enjoy.

Pros. A good nuker late game.
Good vs Other casters.
Low CD on Ulti.
Has good CC's.
Good escaper.
Cons.
Squishy.
Slow
Doesn't get Powerful Till late game.
Uses mana Alot.

Get Null spear first as a good harassing abllity then get nether blade because your gonna need the mana after that go for force pulse and max Null spear geting your Ulti When nessary Get Nether blade lvl 2 when you have 4 Null spear then max null spear and go for Force pulse max again geting your ulti when you can. After that max Nether blade and Then your done :D

First get Doran's ring for the Nice Early game Stats you'll need them because of kassadin being Slighly week Without it early game. Then get Tear of the goddess For the extra mana.
Then get Sorcerer's Shoes For magic Pen and Movement. Next go for Void Staff this will give you some AP and some Magic pen You may chose to sell your Doran's ring Now or wait till you see fit Personally I Sell it here to get enough for Starting to build void staff. Then go for Archangels as an upgrade for your Tear of the Goddess. Then get haunting Guise this will give you some Magic Pen and Some HP Which is helpful. then get a Rabbadons Death cap for Lots of AP And if you would like get another one Or Buy something that Suits it better. Chances are your not going past the 1ist Rabbadons.

Get Clarity for the Mana you'll need early game and Maybe mid game or to give allies Use it near your allies so it'll give them mana to.
And get Ignite for the extra Finishing damage or just extra damage to add to your nuke Works well early game not as much late game.

This has been a fun guide to make.
Hope you guys start pwning it up with Kassadin.

Good luck!