Luther
From Lexicanum
Luther was the man who discovered the Primarch Lion El'Jonson on the world of Caliban. El'Jonson's mentor, saviour, and friend, Luther raised the Lion as if he were his own child. He presented him to the Order, the force of feudal knights on the planet who helped raise him. Sadly, Luther betrayed the Lion, rallying many of the Dark Angels on Caliban to his cause and killing any dissidents. When the Lion returned to Caliban from the Horus Heresy, Luther attacked his fleet. The Lion and Luther engaged in single combat. In this battle Lion El'Jonson wounded Luther but could not bring himself to kill his oldest and dearest friend. Luther however was merciless and sent a sorcerous blast straight towards Lion El'Jonson which mortally wounded the Primarch. At that moment realizing how far he had fallen, Luther began to weep for what he had done to his friend, and the Dark Angels captured him and held him prisoner on the Rock. At this point, the Gods of Chaos scattered Luther's forces, the Fallen, through the warp. The intense battle, however, had made the planet unstable, and Caliban was destroyed.
Luther remains chained within the most secure cell of the Rock, his existence known only to the Supreme Grand Master of the Dark Angels Chapter, Commander Azrael, and those who held the rank before him. He is often used as an oracle to find the other Fallen. Azrael, like his predecessors, has attempted to make Luther repent for his crimes ten millennia earlier - but, like his predecessors, he has failed to do so. Luther only continues to rant and rave, claiming that Lion El'Jonson would come and absolve the Fallen...and that the day of the Lion's return was close at hand.
