Zess are a species creates by mick39.

:FROM THE WIKI:
The Zess are often called the "Living Dead" race of Vilous. The race is comprised of those who have died and been reborn with a Zess body.

All that is known is that in order to become a Zess, one must carry around with them a small object at all times, then, when the person dies, their mind is transferred to a new "Zess body" that lacks body heat, processes food much better than normal bodies, and is said to be entirely synthetic. The Zess body is different in appearance from their old body, but still retains some features of their old race, like an elongated muzzle on Sergals who become Zess.