Revenants are beings that have been reincarnated. Modern day terms would identify them as "zombies", but their intelligence is far more superior. Revenants were originally created by Carnegie, the man behind the slayings of many greek gods thousands of years ago, as a strategic maneuver to increase his victory chances. After the battle, every single revenant died from either being killed by the gods or by the hostile wildlife.
Revenants were originally created by Hades with the power of the Incarcerous, a sceptre that supposedly had the ability to reincarnate the dead, to serve as slaves when all of a sudden they were dying out one by one, presumably due to what modern examinations say are severe brain hemmorhages. Frustrated, Hades cast away the Incarcerous, where it would later be found by Zeus. Discovering its power, Zeus locked it away, never to be used again, until the day one man befriended Zeus. That man was Carnegie, who would later use Zeus's sword to destroy the locks and steal the Incarcerous, to plan for his insidious plot to wipe out the gods. It was unsuccessful.