5. Warhammer 40k factions Chaos demons
Moving on to the main antagonist of K, the forces of Chaos, the K Universe exists alongside a parallel dimension known as the Warp, a dimension of emotional and psychic energy where time and space mean little and where there are things living in the warp. All that emotional energy over time has coalesced into great powers, horrific god-like intelligences that want to break into real space and corrupt the sentient species that live there. This would be fine if the Imperium didn't also rely on the warp for interstellar travel and communications.
Over time, various factions have been corrupted by Chaos, with the most prominent being the Chaos Space Marines. These are the rebels who lost the Great Rebellion, also known as the Horus Heresy, ten thousand years ago. They have been hiding out in the warp ever since, growing increasingly bitter and powerful over time as more Renegade Space Marines flock to their cause. Chaos Space Marine raiding parties are a constant threat in many parts of the Galaxy, but the greater threat comes from the Black Crusades led by Abaddon the Despoiler. During these Crusades, thousands of ancient heretic Astartes are sent on the Imperium, not only with ancient patterns of Space Marine arms and armor but also twisted demon engines that fuse Forbidden Science with the raw stuff of the warp itself.
Like the Space Marines of the Imperium, the Chaos Space Marines can be broken down into various Legions and warbands, each with slight differences. However, there are a couple of unique ones, such as the Thousand Sons, who serve the Chaos-powered Zinch god of magic. Before the heresy, their mastery of the psychic arts was unmatched, but since their fall to Chaos, the majority of the Legion have become nothing more than animated suits of armor enthralled to ridiculously powerful Sorcerers and accompanied by twisted magical beasts.
The Death Guard is dedicated to Nurgle, the god of disease, with their plague Marines being bloated, filth-encrusted parodies of the noble Warriors they once were. They trudge relentlessly forward, spreading viruses and plague wherever they go. Finally, the World Eaters worship Khorne, the manifestation of rage and anger. Their Warriors are barely controllable Berserkers created by brutal psychosurgery, existing only to spill blood in the name of their lord.
Wherever any of the Chaos Space Marines go, a greater threat follows. Once the barriers between the worlds have been thinned, the denizens of the warp itself can break through. These Chaos demons and Nightmares given form are hordes of twisted creatures dedicated to one of the four Great Chaos Powers: Khorne, Tzeentch, Slaanesh, and Nurgle. They sweep in through breaches in real space, damning entire worlds to an eternity of slaughter for the amusement of the Thirsting Gods.
Chaos can corrupt anything, and some of their more powerful Scions are the Chaos Knights, the descendants of Imperial night houses that threw their lot in with the traitors millennia ago. Their Knight Walkers are twisted Horrors suffused with the power of the warp, with their pilots fused into their command thrones or possessed by demons. Like the Imperium, there are also many smaller factions in the background. Corrupted Tech-priests known as the Dark Mechanicum labor in their hell forges to supply the Chaos Marine armies with demon engines, while human cultists or traitor Imperial Guardsmen make up the hordes of the Lost and the Damned.
However, it's not just the armies of the Imperium that are in danger from the warp. Chaos is something of an existential threat to everyone in the universe, and humanity isn't the only target.