Caliban - Destruction of Caliban: After the end of the Horus Heresy during the Siege of Terra, Lion El'Jonson and the Dark Angels fleet finally returned to Caliban, but once the I Legion's vessels made orbit they were immediately fired upon by Caliban's orbital defence system.Puzzled, El'Jonson withdrew his vessels into the outer reaches of the Caliban System. When he learned that Luther had ordered Caliban's defences to fire upon the returning Dark Angels fleet, El'Jonson fell into a terrible rage, believing that Luther and the Dark Angels garrison left on Caliban had fallen to the temptations of Chaos just like the Astartes of the Traitor Legions.The Lion ordered his fleet to launch an orbital bombardment of the planet and destroy the weapons of the orbital defence systems. The Legion's fleet bombarded the planet mercilessly, causing the tectonic plates of the planet to crack and shift. El'Jonson then personally led an Astartes strike force into the Tower of Angels, the I Legion's primary fortress-monastery on Caliban and duelled his former friend and mentor to the death.They were equally matched, but finally, Luther keeled over in exhaustion, leaving him open to a deathblow. Yet for all his wrath, El'Jonson could not bring himself to slay his foster father and so, in that moment, Luther lashed out with a psychic attack using the sorcerous powers that had been granted to him by his willing acceptance of servitude to the Ruinous Powers.Yet as Luther saw his oldest friend dying, his mind finally cleared of its jealousy and hate, as if a veil had been lifted, and he fell to his knees in anguish, sending out a cry for help into the Warp. The Ruinous Powers, angered that they had lost such a valuable pawn in their plans to weaken the Imperium, sent a massive Warp Storm to wrack the surface of the planet.The world of Caliban then broke apart under the strain produced by the Warp Storm and the prior geological stresses unleashed by the Dark Angels' orbital bombardment. The entire world imploded into a field of orbital debris, with the exception of the oldest and mightiest fortress-monastery of The Order, which had been protected by potent defensive gravitic forcefields.Officially called the "Tower of Angels," but better known as The Rock, the Calibanite bedrock upon which the fortress sat was later hollowed out by the Dark Angels, fortified with advanced Imperial weaponry and fitted with a Warp-Drive to transform it into a mobile Star Fortress which would serve as the Dark Angels primary fortress-monastery.Immediately following the destruction of Caliban, when the Dark Angels searched the ruins of the Tower of Angels, they found the Arch-Traitor Luther, babbling incoherently that Lion El'Jonson had been taken by the Watchers in the Dark (supposedly to heal from his psychic attack), but I Legion's primarch was nowhere to be seen, nor could any trace of his body be found where he had duelled Luther.The rest of the Dark Angels Astartes who had been converted by Luther to the worship of Chaos were sucked into the Immaterium by the will of the Chaos Gods and were scattered across time and space. From that time forward they were forever after referred to as the Fallen Angels or simply the Fallen, and are hunted ruthlessly by the Dark Angels and their Successor Chapters, who call themselves the Unforgiven.All bear the haunted legacy of lost Caliban and all have pledged that they shall never rest until every one of the Fallen has repented his sins against the Emperor.