His abilities primary scale with target's health and less with attack damage. Because of that he can build defense and still able to deal decent damage, therefor he can be played as a light fighter who builds both defense and offense. He can be Laner and Jungler
Fai Zen used to be a great Ionian warrior who have fallen into the realm of the Void. With years he strived to find path to his home world, he survived, killed, run and hide. The void changed him into another "higher" being giving him advanced powers he never imagined but also changed his outlooks. Once he was able to returned to Ionia he found his home village ruined. Now he seeks vengeance upon these who are responsible for such monstrous deeds, not only because he is filled with anger, but also sadness knowing that when this happened he wasn't there to defend whats dear to him
Void Cannon can be used to farm lane minions and light harassment as well as utility for slowing down targets which may make them vulnerable to ganks.
Phase Shift either to escape or can be followed by Voidblade for heavy harassment and will be refreshed if takedown is scored.
Voidblade can be used in a VS duel and farm high health jungle monsters and duel tankier champions while giving him light sustain
Hyper Shift is all or nothing ability. It will increase damage output and will boost his mobility allowing him to easy maneuver in teamfights or escape from dangerous situations.
Mark of the Void is his assassination tool. After landing couple of abilities on enemy champions, his next basic attack can trigger the Mark and would assassinate them if they are low enough on health
Assassination combo - begin with Void Cannon then Phase Shift and Voidblade followed by basic attack to trigger Mark of the Void for assassination.
Fai Zen is always about execution therefor he should always be in look out for low health targets desite their amount of protection.
Before entering in teamfight he can harass enemies with Void Cannon and after much Durable champions in his team enter in a fight then he can follow up with his other abilities for assassination of his enemies that are low on percent health. Scoring Takedowns will refresh Phase Shift and if his abilities are not on cooldown he can jump into his next pray