Daemon World
A Daemon World is what happens when the Imperium of Man doesn't stop a Chaos incursion in time.
Under normal circumstances, the forces of Chaos are limited by the inherent instability of daemons: they don't last long outside of the Warp because they are made of emotion and psychic power, and so they slowly start to destabilize and disintegrate as soon as they enter realspace, unless they're fed a constant stream of sacrifices (the method of which, is dependent on the daemon's alignment). Some can stick around longer if they possess an individual, but that takes time and leaves them relatively vulnerable, as in addition to having their abilities reduced in power, having the host destroyed or captured can severely turn the tables on them.
The ultimate solution to this problem, and the ultimate goal of most Chaos cults, is to bring the Warp with them, partially merging it with realspace, which allows daemons to cross over freely.
This has the unfortunate side effect of breaking reality: the chaotic, anarchic influence of the Warp causes the laws of physics to become more like guidelines, and daemons are seldom inclined to follow the rules. Under typical circumstances, most soul-bearing beings trapped on a Daemon World will die from being exposed to corrupting warp energies, have their souls rent from their bodies from being exposed to the immaterial, or simply being massacred by it's new inhabitants, and their souls consumed by the various entities in this new-found hell. If the daemons are feeling particularly cruel, the inhabitants will instead survive, but slowly driven insane by the warp's influence overtaking them (whether it is a good pain is dependent on the individual). After a while, the surviving populace will be at the mercy of the planet's new inhabitants, either being slaves, sacrifices, and/or playthings for entertainment. That said, even dying in a daemon world is a shitty gig; the moment your soul is released from your mortal coil, you can count on being eternally tormented and by billions of neverborn inhabiting unreal-space.
For beings without a soul (hence no warp signature), they're immune to the influences of the warp, but they're still vulnerable to having various sharp objects rammed down their faces, as the denizens of the warp overtake their planet.
Because losing a world to the Warp means death or worse than death for all of its inhabitants and strengthens their enemy at the same time, the Inquisition has concluded that, should all preventative efforts fail, it is more merciful to commit Exterminatus upon a planet about to become a Daemon World than to allow the heretics to complete the necessary rituals. That said, most Inquisitors would only resort to this, if the situation is indeed unsalvageable, as destroying your own territory will hurt you more, than it will your enemy. Once the world is successfully transformed, though, it's a safe bet Exterminatus wouldn't work on it in best case scenarios. At worst it could make daemonic inhabitants of the world even stronger.
Daemon Worlds
- Most planets in and on the rim of the Eye of Terror,
with the exception of CadiaWHOOPS. - Sicarus, ruled by Lorgar and homeworld of the Word Bearers. Not to be confused with Cato Sicarius or the Ordo Sicarius, which are (probably) not ruled by Lorgar.
- Medrengard, ruled by Perturabo and homeworld of the Iron Warriors.
- The Plague Planet, ruled by Mortarion homeworld of the Death Guard
- Sortiarius, the Planet of the Sorcerers, ruled by Magnus the Red and homeworld of the Thousand Sons
- The Pleasure Planet, reportedly ruled by Fulgrim and would-be homeworld of the Emperor's Children, if they could find it
