From Lexicanum

A daemon world is a planet within the Eye of Terror ruled by a Daemon Prince.

Daemon worlds are not bound by the laws of physics, but by the whim of the Daemon Prince.

Each of these worlds is a bizarre sub-reality ruled by terrible Daemon Princes. On these worlds Chaos reigns triumphant. The four Chaos Gods continuously compete to possess the Daemon Worlds. Armies of daemons and their living allies fight huge and bloody battles to determine which of the Chaos Powers will posses them. These battles often last for hundreds of years, so that the entire world becomes little more than a gigantic arena where the opposing forces are pitched against each other. The Chaos Powers do not, of course, appear in person to lead their armies - they are spectators to events, not participants. Their generals are Greater Daemons and favoured Daemon Princes who, because they were once alive, understand the nature of both the material universe and the immaterial Realm of Chaos. Once a Daemon Prince has conquered a world, his grateful Patron gives it to him as a gift to rule over as he wishes.

When a Daemon Prince takes control of his hard-won world he uses his mighty powers to reshape it to a form which pleases him. Because of this, every world is different and all are equally spectacular in their own way.

The most powerful psykers have reported dreams or visions in which worlds of the Eye of Terror have been revealed to them. On one world a black sun stands in a white sky and smoky threads pour from it onto a tangled black city - this is said to be Medrengard, the world of the Daemon Prince Perturabo, formerly the Space Marine Primarch of the Iron Warriors. Another world has boiling lakes of blood from which spheres of fire float into the sky and spread their light across the firmament - the ruler of this world is the Daemon Prince Bubonicus, formerly a mortal Champion of Nurgle on one of the myriad lost worlds in the galaxy. Visions of such places disturb the psychically sensitive throughout the entire galaxy.

Notable Daemon Worlds

• Bubonicus
• Eidolon
• Medrengard - post-Heresy world of the Iron Warriors
• The Plague Planet - post-Heresy world of the Death Guard
• Planet of the Sorcerers - post-Heresy world of the Thousand Sons
• Torvendis- World at the very center of the Maelstrom
• Draakasi- Within the Eye of Terror dedicated to the worship of Khorne

Worlds of the Galaxy
Imperium	Adeptus Astartes Homeworld • Agri World • Cardinal World • Civilised World • Dead World • Death World • Desert World • Feral World • Feudal World • Forge World • Garden World • Hive World • Ice World • Industrial World • Jungle World • Mining World • Ocean World • Paradise World • Penal World • Research Station • Shrine World
Xeno	Xenos World • Daemon World • Exodite World • Tomb World • Maiden World • Crone World • Ork World • Tau Empire • Craftworld

Sources

• Realm of Chaos, Slaves to Darkness 1988, and The Lost and the Damned 1990