"Amongst the canon of the Priesthood of Mars, the Anima Mori are considered the most reviled, dangerous and oldest of the tech-heresies. Their creations often manifest themselves as reanimated beings that have been brought back from death itself but are now filled with unnatural life. Such beings are known as corpse-thralls or carrionates. Although the distinction between a corpse-thrall and a Servitor may seem a fine one, there is a large difference between the two. For one, servitors are created from recycled living tissue or even vat-grown synthesized tissue, which is an accepted part of Imperial technology. Furthermore, the use of preserved organic components such as bone is also part of the servitors' construction.
Anima Mori, however, go further than accepted practices as they seek to reanimate the dead and even necrotic tissue in order to create an undead servitor that is beyond the frailty of flesh. In theory, this in effect creates an immortal living machine that is free from the limitations imposed by death."
Source - Dark Heresy: Disciples of the Dark Gods, pg. 62