Luther

Luther, also known as the 'Arch-Betrayer', was an acclaimed knight of a monastic group known as the Order that discovered the young Primarch Lion El'Jonson living in the forests upon their deadly world of Caliban. He became the Primarch's saviour and took the man born of the forest with them back to civilisation. He raised the young Lion as his son, becoming a mentor, friend, and eventually as a brother to the remarkable young man. When the Emperor of Mankind came to the world of Caliban reunited with his long lost son, he gave Lion El'Jonson control of the Dark Angels Legion, sending him on his way to continue the Great Crusade across the galaxy. Ironically, Luther would eventually betray the Lion, rallying a good portion of his fellow Dark Angels to his cause and attacked the Lion's fleet as he returned from the Horus Heresy. The titanic struggle between the two former friends resulted in the destruction of Caliban and loss of the Dark Angels' Primarch.