Sortiarius
Sortiarius (roughly the Latin word for "Sorcerer"), also called the Planet of the Sorcerers, is the Daemon World that serves as home and headquarters of the Thousand Sons Legion after the Burning of Prospero, which, evidenced by that battle's name, left their old home of Prospero destroyed by the Space Wolves. Sortiarius was a Daemon World in the Eye of Terror, and Tzeentch had put the "Reserved" sign down on it when he first got his hands/feathers/tentacles on it.
The arrival of the Thousand Sons was more or less by accident (but not really), caused by being chased from their old home by Loyalist allies after they faithfully did their duty to the Emperor (but not really). When driven from their home, Magnus had his surviving sons and what remaining humans they could take with them into the central pyramid, then he said the magic word, and the whole damn building and the surrounding districts were thrown through the warp and ended up on a strange new world. Once they set foot on the planet, the Thousand Sons found that their powers had expanded, increasing their psychic potential and finding that they now had access to powers outside of the domain their specialized cult. It also meant that mutation was a major problem - until Ahriman took charge of the situation, but that's a different story.
When the Thousand Sons arrived, Sortiarius was uninhabited, but they eventually built it up with materials and slaves taken in raids across realspace, now settled with a twisted mockery of their old home city of Tizca. Every Thousand Son who could be bothered (i.e. only the Sorcerers) now has his own Wizard's Tower. The main population is comprised of humans, the descendants of the original Prosperine humans, who still serve as the Sons's servants and occasionally in the Spire Guard auxiliaries, and the slaves that have been taken through raids into real space over the years (though calling any group but for the freshest of captives "human" is probably inaccurate). Most of the humans that live on Sortiarius have mutated into Beastmen (IN SPACE), though for the most part they are a lot more civilized and smarter than the Beastmen of WFB. Most of the Tzaangor (beastmen of Tzeentch) are "wild", living and preying on the blasted planes of Sortiarius, but Sons do occasionally recruit/press gang/enslave some to serve as enforcers and crew on their ships. Tzaangor are actually quite good at it by virtue of being much more intelligent and disciplined than most other beastmen - Abaddon himself recognized Sortiarius Tzaangors as some of the best shipwrights of the Eye. Assuming their social structure is anything like Prospero's, aside from the mutations, life is probably better for the humans, mutants, and beastmen on Sortiarius than on just about any planet in the Imperium.
The world is ruled by Magnus, who has the biggest, blackest tower, and it has a giant eye hovering above it. We would wonder if the writers were even trying, but this was written before the LotR movies, leaving only the more credentialed neckbeards to wonder if the writers were even trying. Back on topic, Magnus is like most of the other traitor Primarchs, who are the daemonic equivalent of the guy who rarely ever gets off of his couch except for when he really needs to get shit done. He spends most of his time playing the Great Game, leaving him to plan and scheme and occasionally yell orders to the Thousand Sons who've stayed on Sortiarius.
Sortiarius also likely holds quite a few treasure troves of knowledge, arcane artifacts, strange valuables, and lots of otherwise weird shit. They also do low key recruiting there, training any young, prospective psykers and weeding out all but the physically and mentally most capable, and elevating them into Sorcerers. It's also hypothesized that they resurrect fallen Rubric Marines, keeping their dead from resting and shackled to the will of their former brothers, but then there's always been as many Rubric Marines as needed, as long as the writers had Thousand Sons Sorcerers who needed minions, so take that for what it's worth.
Not much is known about Sortiarius beyond that, but there are some known details. Want Wizard's Towers? They got those. Want mutants? They got those. Want a sky made of fire the color of a kaleidoscope on crack? Want ambiance made of the screams of the damned? No? They got that anyway. The planet even has a ring made of souls who died by deceit. Yeah. It's a pretty fucked up place. And there's plenty more weird shit to go around.
Most of this is mentioned in passing by figures like Iskandar Khayon and Ahzek Ahriman, or from a Thousand Sons short story: there've always been few references and only oblique descriptions of Sortiarius. Oh, and it was only called "The Planet of the Sorcerers" until Aaron gave it a proper name in 2014. Graham McNeil, however, doesn't use it, even stating that to the Thousand Sons no name for the world sticks for very long.
We get a brief glimpse of Sortiarius in the Wrath of Magnus trailer (though sadly, everything is rendered black and white).
As of Wrath of Magnus, it is now in realspace around Prospero, bringing with it all the warp shit you'd imagine and making the galaxy flooded enough to keep certain Daemonic entities stable.