Daemon (Warhammer 40,000)

Daemons, also called Neverborn, are entities of Chaos, servants of the Chaos Gods native to the Warp. They are formed by the will of the Chaos Gods and act as extensions of the gods' wills, lacking real free will as most people would think of it. A daemon's appearance and character reflect those of the god that created it. Daemons can be reabsorbed into the god at the deity's whim, which is the only way they can truly be killed.

Daemons in realspace
Daemons cannot exist independently in realspace for very long, as they are fundamentally creatures of the Warp. They can, however, access realspace in a variety of ways, some which allow them a more permanent foothold.

Possession
Daemons can infiltrate the minds of psychically sensitive mortals. Initially, this starts off as a slow and subtle subversion, but eventually the victim's mind is subsumed by the daemon and its body transformed into a shape of the creature's liking.

Daemonic incursions
Daemons can spill into realspace through a Warp rift or an opening created out of a possessed psyker. This allows them a link back to the Warp, providing them the energy they need to stay manifested in realspace.