The Necromorphs are reanimated corpse soldiers utilized by the Terrasa Regents.
When you need one, just thaw him out.
Accurate information on Necromorphs is hard to come by because the Terrasa Regents keep the real stuff secret. The information they release is mostly exaggerated old wives tales. All this stuff about how the Necromorphs can be buried for years and then awoken to reek havoc is shear nonsense. The Necromorphs must be kept frozen until use. Warming up takes about 6 hours (terran time). For this reason The Terrasa Regents hire Fabian and Eli mercenaries to man their permanent bases and fortifications. They would hire Amerons (they work cheaper) but for the fact that the Amerons consider the Necromorphs and their creators as blasphemers against nature.