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.

Daemon Princes
Daemon Princes were once the most powerful mortal servants of Chaos. As a reward for their service, they have been transformed into true daemons of great power. Each Daemon Prince is a unique entity with its own set of powers.

Greater Daemons
Greater Daemons are extremely powerful daemons that occupy the upper ranks of Chaos. Greater Daemons serve as guardians of their god's domain, as commanders of great daemonic hordes, and as sometimes even as avatars of their gods. Greater Daemons are beings of such might that they cannot simply enter the physical world, and require a possessed host body to manifest themselves.
 * Bloodthirster, Greater Daemon of Khorne
 * Great Unclean One, Greater Daemon of Nurgle
 * Keeper of Secrets, Greater Daemon of Slaanesh
 * Lord of Change, Greater Daemon of Tzeentch

Lesser Daemons
Lesser Daemons form the backbone of Chaos' armies as its foot soldiers. They are generally vaguely humanoid in appearance, if monstrous in form, and cruelly intelligent. Lesser Daemons are much more able to enter realspace than more powerful daemons, and consequently are the most common variety of daemon to find there.
 * Khorne: Bloodletters
 * Nurgle: Plaguebearers, Nurglings
 * Slaanesh: Daemonettes
 * Tzeentch: Horrors, Flamers

Daemonic Creatures
Daemonic Creatures are the same sort of entity as other daemons, but they are animalistic and non-sapient. They can be further divided into Daemonic Beasts and Daemonic Steeds.

Daemonic Beasts
Daemonic Beasts are animal-like daemons, generally used for hunting or for warfare.
 * Khorne: Flesh Hound
 * Nurgle: Beast of Nurgle
 * Slaanesh: Fiend
 * Tzeentch: Screamer

Daemonic Steeds
Daemonic Steeds are daemons used as mounts either by mortal servants of Chaos or other daemons.
 * Khorne: Juggernaut
 * Nurgle: Rot Fly
 * Slaanesh: Mount of Slaanesh
 * Tzeentch: Disc of Tzeentch