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. Some can stick around longer if they possess an individual, but that takes time and leaves them relatively vulnerable. 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. Humans trapped on a Daemon World will die instantly if they are lucky, but the unfortunate survivors will inevitably be driven mad by the Warp overtaking them, and used as slaves or toys by the occupying daemons.
Because losing a world to the Warp means death or worse than death for all of its inhabitants, 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. 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.
- 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.
- Medrengard, ruled by Perturabo and homeworld of the Iron Warriors.
- The Plague Planet, ruled by Mortarion homeworld of the Death Guard
- 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