Kassadin

Strengths

Kassadin has multiple item paths; he can go caster via Mana and Ability Power or anti-caster with Cooldown Reduction and Magic Resist.
Kassadin's ultimate has many uses and is on a shorter cooldown than most, use it often.
Try to get the Ancient Golem buff to counteract Riftwalk's increasing Mana cost.

Passive · Void Stone — Kassadin takes reduced magic damage and ignores unit collision.
Q · Null Sphere — Kassadin fires an orb of void energy at a target, dealing damage and interrupting channels. The excess energy forms around himself, granting a temporary shield that absorbs magic damage.
W · Nether Blade — Passive: Kassadin's basic attacks deal bonus magic damage. Active: Kassadin's next basic attack deals significant bonus magic damage and restores Mana.
E · Force Pulse — Kassadin draws energy from spells cast in his vicinity. Upon charging up, Kassadin can use Force Pulse to damage and slow enemies in a cone in front of him.
Ultimate · Riftwalk — Kassadin teleports to a nearby location dealing damage to nearby enemy units. Multiple Riftwalks in a short period of time cost additional Mana but also deal additional damage.

A suggested 5-lane lineup built around Kassadin, based on hero partnerships and role synergy — not win rate.