During the Great War, Zeratul was a tutor to Tassadar in the ways of the Dark Templar. Called to the Imperial Sanctum by Tassadar's phantom, Zeratul arrives to aid his pupil however he can.
While Zeratul suffers from his small health pool and melee range, his ability to secure ganks and pick off squishy heroes with his ultimate allows him to be very versatile in teamfights due to the Chronosphere.
Cool video depicting the evasiveness of Zeratul
With two skills that can be as harmful as hurtful to your team if used improperly, Zeratul is a high burst autoattack carry/initiator with large potential to succeed or fail spectacularly. Mastering use of his E is absolutely vital, as it proper positioning with his R to avoid teammates. He is also rather squishy, and is best suited to counter engage unless a large opportunity presents itself. Max Q first for aggressive play, or W for ease of denying. Run 2/1/3 talents, taking attack damage, attackspeed, health, cooldown reduction, energy regen, and movespeed.