Planet of the Sorcerers

The Daemon World of Sortiarius, known also as the Planet of the Sorcerers, is the current homeworld of the Thousand Sons Traitor Legion. The Planet of the Sorcerers is a Daemon World located deep in the Eye of Terror. With the fall of his original homeworld of Prospero to the Space Wolves at the start of the Horus Heresy during the Scouring of Prospero, the Primarch of the Thousand Sons Legion, Magnus the Red, used his sorcerous powers in the final act of his pact with Tzeentch to transfer his beloved and infamous City of Light, Tizca, through the Warp from Prospero to a new planet so that all of the precious (and heretical) knowledge hidden within its libraries that the Thousand Sons had gathered from all across the galaxy during the Great Crusade would be preserved.

This planet is now a Daemon World of Tzeentch ruled by Magnus as its Daemon Prince; it is dark, rocky, highly volcanic and suffers from constant climatic turmoil and change. Its skies are scarred by relentless storms of Warp energy. Towers thrust up from rocks jutting from the world's lava plains, twisted mockeries of what the City of Light once was like in the years of the Great Crusade. Dwarfing all the other edifices is Magnus' own tower, an obsidian monolith called the Tower of the Cyclops. The great, glowing eye of the Tower watches over the planet, its gaze penetrating the Warp into the material galaxy beyond the borders of the Eye of Terror. This allows Magnus to sense the arcane artefacts and gifted psykers which fascinate him, so that he can dispatch his Thousand Sons through the Warp to raid the Imperium of Man.

When the Thousand Sons first arrived on the planet, they found it unpopulated. Over the millennia, however, the Thousand Sons have acquired tens of thousands of human slaves through their raids into realspace. This has created a servant underclass on the world, some of whom have escaped their masters to eke out a miserable but free existence in the hostile land surrounding Tizca. These escaped slaves then join the nomadic warbands that roam the Planet of the Sorcerer's wastelands and are led by both sorcerous Champions of Chaos from the Thousand Sons as well as those chosen few of the Lord of Change who have arisen within the ranks of the escaped slaves. Often these Chaos Champions are recruited from these nomadic warbands to serve the Thousand Sons. Over thousands of standard years, a large population of mutant Beastmen has also developed on the planet, forming a subordinate warrior caste for the Thousand Sons Legion. While some of these mutants -- specifically referred to by the Thousand Sons as Tzaangors -- serve various warbands, most remain "wild." Little else is known of the other flora or fauna present on the world, apart from the presence of Chaos Hounds amongst some of the roaming warbands.

Trivia
The Eye of Magnus atop the Tower of the Cyclops is similar to the Eye of Sauron atop the Barad-Dur, the Black Tower, in the Lord of the Rings by J.R.R. Tolkien.