Daemons in Warhammer 40,000
In the Warhammer 40,000 universe, daemons are entities which live in the parallel dimension known as the Warp. Similar to the Realm of Chaos, this parallel dimension is made up of pure psychic energy, and this energy sometimes collects and forms living entities. As these entities gather more energy, they can become sentient, eventually even god-like in their powers, abilities and ambitions. Like their Fantasy counterparts, daemons are the lesser entities serving the Chaos Gods.
Warhammer 40,000 daemons are normally unable to enter into the physical world on their own; as beings of pure energy, they must be summoned to the battlefield by Chaos worshippers and cultists. Alternatively, they can enter the physical realm at overlaps of Warp and material space, such as the Eye of Terror, through certain gates between the material and Warp realms or by forcing itself into the mind of a psyker, especially untrained psykers.
Once in the physical realm, daemons are often found fighting alongside Chaos worshippers- mainly the forces of the Chaos Space Marines, but also Chaos-influenced Imperial Guard and rebels known as the Lost and the Damned. Less frequently, other races such as the Orks (it is hard to tell) or Eldar can be corrupted by the Chaos Gods. When this happens, they too can be seen fighting alongside daemons, although not necessarily those of the four major gods. Like Fantasy daemons, they are sometimes difficult for physical weapons to destroy, but certain specially prepared weapons do exist which can more easily defeat them, banishing them back to the warp (or even obliterating them from existence); like their Fantasy counterparts, if they are defeated in combat, the power holding them in the physical universe can begin to fade, banishing them back to the warp. (On a side note, the current release of the Codex: Chaos Space Marines has removed the "instability" rule, thus daemons are no longer in danger of dissipating in battle.)
While in the Warp, daemons are attracted to the souls of mortals, which they like to consume, and this can aid in the process of summoning them, as they will flock to a battlefield in order to devour the souls of the recently slain. Due to their hunger for mortal souls, they are known to make the use of psychic powers by so-called "psykers" dangerous. When a mortal psyker uses his powers, he draws upon the energy of the Warp, and daemons are strongly attracted to this usage of Warp energy. Once it finds the psyker, the daemon will then attack his mind in order to devour his soul or possess his body in order to gain access to the material world. Some races' souls are more attractive to daemons, notably the psychically powerful Eldar, and these races must make use of certain rituals or equipment to protect themselves from daemonic attack. Extremely powerful psykers (on the level of the few children/grandchildren spawned by the God-Emperor) should be able to obliterate daemons from existence (both the physical body and the psychic energy). Special psykers called Blanks exist that actually block and even destroy psychic energies. In theory a daemon whose physical body is destroyed in the presence of the Blank would be permanently killed.
Lastly, some races, mainly the Imperium, are able to fly their ships into the Warp and make use of the Immaterium as a form of hyperspace in order to quickly travel across the galaxy at faster-than-light speeds. While traversing the Warp, these ships must make use of powerful protections like the Gellar Field lest they be attacked and their crews consumed by the waiting daemons.
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