A Zompire is the result of someone infected with the vampire virus receiving the zombie virus, resulting in an equilibrium between the two clashing afflictions.
The first Zompire was Andrius, who, while in a weakened state, was attacked by zombies. He then infected Lieutenant Cross, who in turn infected Dominik. Since then various other humans have been infected, most now serving Dominik.
Zompires share all of the characteristics of normal vampires (having the normal heightened abilities but also the weakness to sunlight) as well as some stronger abilities. Unlike vampires (who are generally proud of their state) Zompires appear to be afflicted by a certain self loathing. Andrius often refers himself as an abomination, Cross seems to revel in masochism while Dominik actively seeks a cure to his condition.