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. Before the discovery of the Lion, Luther was the foremost hero of his times. 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.*1
After the Emperor
While crusading with Lion El'Jonson against the Sarosh Xenos, he discovered a bomb aboard the ship which would kill the Lion. Initially his jealousy encouraged him to leave onboard and let the lion die, however his better nature kicked in and he disabled it, with the help of the librarian Zahariel, saving the Lion's life.*4
Controversy
In the novel, Angels of Darkness by Gav Thrope, a different version of Luther's Betrayal is revealed. In the book, Astelan, an original member of the Dark Angels who was transported several millennia into the present when the Warp Storm tore apart Caliban, tells this unorthodox version of the events during the Betrayal. Astelan claims that it was the Lion who betrayed the Dark Angels, not Luther. His claim is considered heresy by Interrigator-Chaplain Boreas, and Commander Azrael had Astelan placed in a cell near Luther.*2
However in the novel Fallen Angels, Luther is the one who secedes from the Imperium, claiming the Lion had betrayed them by leaving them on Caliban for centuries, while the Great Crusade continued. His superstitions where furthered when he learnt of a cult of Terrans where in the process of a daemonic ritual (which was narrowly stopped by Zahariel, a librarian who Luther had grown fond of). He took to reading the books of an order who studied the Warp. He put what he learnt to practice- an example being when the Librarian Israfael attacked him with a bolt of lightning, it simply dissipated.*3