Inwit
Inwit is a tidal-locked ice-world. It rotates on its axis at the same rate it rotates around it's sun (like how the Moon rotates around Earth), meaning that one half is permanently bathed in sunlight, and the other half is also dark (apparently fairly common for settled worlds). While less in the spotlight than, say, Fenris, Inwit is also an icy hellhole where living another day on it is a feat unto itself.
Hmmm?
Oh, that guy.
It's also the original homeworld of Rogal Dorn. Dorn landed among the ice-hives of the warrior caste, and was taken in by the Patriarch of House Dorn, who raised him like a grandson. Over the next few decades Dorn became Warlord of House Dorn and Emperor of Inwit. The shipyards of Inwit were also the place where The Phalanx was built (according to some stories). Following the Emperor of Mankind and Dorn meeting somewhere in space (Rogal was the only Primarch who actively sought out the Big E, but hey, he had the tools for the job), Inwit became the Imperial Fists' homeworld, recruiting ground, and main dry dock. The shipyards of Inwit were dedicated to building battle barges and strike cruisers for the VII Legion. Anyone who wanted to join the Imperial Fists had to dress in rags and walk across the Splintered Lands on Inwit's nightside while avoiding bear-like monsters, which proved you were masochistic determined enough to join the Fists.
Their people are known for their sewing ability.
Following the Ullanor Triumph and the Emperor's decision to return to Terra, Dorn was charged with building the fortifications of the Imperial Palace, and the Imperial Fists' homeworld was officially transferred from Inwit to Terra. Following the Horus Heresy, the Imperial Fists official homeworld is still Terra, but for all intents and purposes the chapter is fleet-based, with the Phalanx serving as their mobile fortress-monastery. Inwit remains one of the Imperial Fists' primary recruiting worlds, but shares that distinction with a number of worlds, including Terra and Necromunda.
The current Mistress of the Administratum, Violeta Roskavler, is from Inwit.