Baal
Baal is a radioactive wasteland of a Death World, famous for being the home of the Blood Angels, the most badass vampires in history.
The Blood Angels' Primarch, Sanguinius, fell upon Baal Secundus after he and his brother Primarchs were scattered across the galaxy by the machinations of the Chaos Gods.
Overview
Baal has two moons, called Baal Primus and Baal Secundus. Apparently, the moons were pretty damn close to some Paradise Worlds, with cultures comparable to High Roman Catholic nations but the Age of Strife slightly altered that. With nukes. By the time of the coming of Sanguinius, Baal and its moons were a Mad Max wonderland dominated by a tribe known as The Blood and foul Mutant hordes. Thanks to the effort of Papa Sang, the mutants were defeated and the people of The Blood came to ascendancy. Later during the Horus Heresy, Baal became a central loyalist hub under the control of Warden Arkhad. Many members of the Shattered Legions took refuge on Baal.
The moons' populations now spend their time trying to evolve out of the Stone Age.
Or they did, until the mother of all clusterfucks ensued. Baal Primus was scoured of life by both the Tyranids and the forces of Ka'Bandha, and Baal Secundus was barely any better, but with the arrival of the Indomitus Crusade and Guilliman naming Dante as the supreme commander of all Imperial forces in Imperium Nihilus, things are starting to look up.
The Bulwark to Imperium Nihilus
As promised by Guilliman, a contingent of the Adeptus Mechanicus stayed behind to both rebuild Baal's shattered defenses, and make good on turning Secundus back into a paradise. Baal itself is undergoing a major change, with the area around the Blood Angels fortress monastery slowly becoming a massive city to house the influx of Imperial bureaucracy and soldiery that were streaming into the system.
Even Skyfall, the orbital docks high above Baal, was being expanded to accommodate the vast array of warships that were rallying there, as well as fortified to become one of the most heavily-defended anchorages the Imperium has ever seen.