This is a top lane guide for Kassadin. Works pretty well for me in most games. Since the top laner will build tons of Magic Resist against you, you get tons of Magic Penetration, to make their purchases NULL and VOID.

The items you will grab this game are mainly for bypassing the enemy's Magic Resist and bursting quickly, as well as escaping quickly. Haunting Guise is a cheap and easy way to add a ton of burst damage. The two Doran's Rings along with your W's passive will make sure you don't run out of mana at any given point in the laning phase. The Athene's will also contribute to this cause late game.

Max Null Sphere first for obvious reasons. Burst their HP and prevent them from retaliating with any CC. Great harassment. W is next, mainly because you won't want to spend too much time next to your enemy to Auto-attack them. Your W is mainly useful for picking at their HP after your burst combos. Grab Riftwalk whenever possible since that will be extremely useful for getting out of teamfights that you are being focused in.

Pros

You can kill.
You can escape after level six easily.
YOU. CAN. KILL.

Cons

You are pretty squishy.
Kinda mana hungry early on.
No escapes pre-six.

Play passively in the top lane at first, only harassing with your Q, as most tops can take you out pretty easily if they have Gap Closers or CC. Say you're up against Darius: He can pull you, slow you, spin-slash you, Ignite, and Flash after you to finish you off at level three. Stay away from such situations. Instead look for openings and just poke. At level 6, you can travel to mid or bot to gank and put your other lanes ahead. Kassadin can bypass a lot of the wards that they will place, such as the ones in the river, so you can have a ball snowballing bot and yourself. The main goal here is to STAY SAFE EARLY GAME.