Caliban

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.

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. When the Emperor came to bring the planet into the fledgeling Imperium and place El'Jonson in charge of the First Legion of Space Marines, these orders were quickly folded into the renamed Dark Angels.

Unfortunately, these beasts were also helping to siphon off excess Chaotic energies, 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.

Lion left with most of the Legion, later sending Luther and the remainder back to run the fortress-monastery of Aldurukh after the Sarosh Compliance, but when he returned just after the Horus Heresy...we're not sure what happened. The Dark Angels claim that 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. The so-called Fallen Angels tell a different story: that it was Lion who was the traitor -- either a pawn of Chaos, or at least holding his forces back from much of the Horus Heresy until he could tell who was winning (and then side with them).

Either way, the bombardment, compounded by a miniature Warp storm unleashed by the traitors (wherever and whoever they were), caused Caliban to break apart; the largest piece was that which contained the fortress-monastery (whose reinforcements 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, either becoming full Chaos Space Marines or becoming thoroughly disgusted by both Chaos and the Imperium and going renegade.