Caliban (Warhammer 40,000)
Caliban was the homeworld of the Primarch Lion El'Jonson and, from its admittance into the Imperium of Man until the Horus Heresy, the primary recruiting world of the Dark Angels Legion of Space Marines. It is likely named for the barbaric savage of the same name in Shakespeare's The Tempest.
History
When the infant Lion landed on Caliban, it was a densely forested Death World home to several orders of Knights who protected the commoners from the dangers of the forest. What people didn't know was the planet's proximity (in a galactic sense) to the Eye of Terror had left the planet Warp-tainted, resulting in various Chaos mutants known as "Great Beasts". Lion's adoptive father Luther was a member of one of these knightly groups known as "the Order", and El'Jonson was raised by the Order, eventually rising all the way to the top. El'Jonson then united the planet and killed all of the Great Beasts. When the Emperor came to bring the planet into the fledgling Imperium and place El'Jonson in charge of the First Legion of Space Marines, these orders were quickly folded into the renamed Dark Angels.
Lion left with most of the Legion, later sending Luther and around five hundred others back to run the fortress-monastery of Aldurukh after the Sarosh Compliance. Luther and his companions were notably put out by being left on the sidelines of the Great Crusade, but started the busywork of training new Space Marines and sending them off in their places, refining the process so that it only took two years to generate a fully trained Astartes warrior, and were capable of raising around five thousand of them in each training cycle.
Unfortunately Luther started reading into all of the accumulated Chaos lore that had been collected by the Knights of Lupus, who were one of the knightly factions who opposed the Lion originally. That, coupled with domestic problems at home meant that he soon put a stop to sending those Astartes on the Crusade and kept them in the system and declared martial law. Unfortunately, the Great Beasts that the Lion killed were also helping to siphon off excess Caliban's Chaotic energies, and keeping humans away from the worst of it, and in their absence the... thing that lived in the planet's core was ready to wake up. Naturally, its first target was Luther and the other Dark Angels on garrison duty.
Shit started going south and people's lack of understanding of warp and chaos phenomena meant that no-one really knew what was the cause. Coupled with attempted interventions from the Imperium such as, Imperal "cultists" trying to seal away Caliban's daemonic forces but inadvertently looking like they were trying to summon it instead, and a Knights-Errant investigation which accidentally killed someone could only ramp up the paranoia even further and make things look worse.
The state of alarm, combined with some of the First Legion members own resentment at being left behind meant that there was soon an Astartes uprising on the planet who tried to claim the Astropath tower and get a message out, only to be betrayed and imprisoned in the dungeons beneath Aldurukh.
Luther then decided to cast off the Imperium and make Caliban independent. Things came to a head when a Dark Angels Legion ship came out of the blue to claim the missing warriors that Luther had failed to send, but those Dark Angels were also betrayed and had their ships claimed for the Luther's own fleet, which started expanding outwards and conquering nearby systems under the name of "The Order"..
Typhus of the Death Guard also showed up, and since Luther had already turned his back on the Imperium, he figured he'd make friends with their enemies and give them somewhere to berth and make repairs.
Fall of Caliban
After the Heresy, the Lion and the Dark Angels showed up, after having not heard from Caliban for over a decade and none-the-wiser about what had been happening at home. They were fired upon when their ships entered Caliban orbit, and they quickly determined that Luther and his Dark Angels had fallen to Chaos. The orbiting Dark Angels responded by sending Lion to confront Luther while their ships opened up with an orbital bombardment.
Either way, the bombardment, compounded by a miniature Warp storm unleashed due to third-party time-travel shenanigans from the 41st millennium from various factions including the Death Guard, Fallen Angels and the Dark Angels themselves caused Caliban to break apart; the largest piece was that which contained the fortress-monastery whose reinforcement meant that it withstood the attack and shielded the surrounding rock, which the Dark Angels re-captured and used as their headquarters, calling it The Rock.
The Angels on the surface were scattered through the Warp and became the Fallen Angels. Many became full Chaos Space Marines, though many more become thoroughly disgusted by both Chaos and the Imperium and went renegade.
As of 8th Edition, it's revealed that a continent-sized chunk of Caliban survived relatively unscathed and escaped the system somehow, and is housing a small legion's worth of Fallen.