Pandoraque
The Pandoraque is an ancient Eldar device that if activated on a planet will boost its life-force to incredible levels.[1] It exists to restart the life cycle of the world in the event of its untimely demise through seismic or extraterrestrial means.[2] If activated on a world of living beings this will cause even those who have suffered the most horrific of injuries to not be able to die, and the Pandoraque's victims will be left craving a release from their constant pain.[1]
The Pandoraque was nearly released upon Equinox by a Slaaneshi cult, where it's Imperial population would have been reconstituted into violent never-dead hoards of anguished pain.[2]