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.
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 Primach who actively seeked Big E, but hey, he got tools for the job) Inwit became the Imperial Fists homeworld, recruiting ground, and dry dock. The shipyards of Inwit became dedicated to building battle-barges and strike cruisers for the VII Legion. People 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.
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 Imperial Fists are fleet-based with Phalanx serving as their mobile fortress-monastery. Inwit remains one of the Imperial Fists primary recruiting worlds, but holds that distinction with a number of worlds, including Terra and Necromunda.