A daemon is a living entity formed from the energy of the Warp by the thoughts and feelings of living beings in normal space. They can materialize in normal space via special psionic rituals or sometimes by exploiting careless psykers.
Daemons can possess a variety of supernatural abilities, including the ability to influence human behavior, alter the biology of living creatures, and manifest directed-energy attacks. They tend to be resistant to physical weapons, but vulnerable to psychic abilities.
Daemons manifested in normal space are inherently unstable and typically dissipate or return to Warp space after a period of time. The more intelligent varieties of daemon can possess living bodies, allowing them to remain in normal space indefinitely, as long as the host lives.
Daemons vary widely in intelligence, strength, and behavioral tendencies. Most daemons have some affiliation with one of the four major Chaos Gods that dominate the Warp.