During the Horus Heresy, Lion El'Jonson, the primarch of the Dark Angels, fought in the Thramas Crusade and then aided his brother primarchs Roboute Guilliman and Sanguinius in administering the ill-fated Imperium Secundus on the far side of the Ruinstorm from Terra. The Lion also served as the captor of his brother the Traitor Primarch Konrad Cruze for a time and aided his brothers in defeating the Daemonic forces that blocked the Blood Angels' path to Terra at the Second Battle of Davin. After the end of the Heresy, the Lion was betrayed and duelled with his second-in-command, Luther, on their homeworld, Caliban before it was destroyed by the Dark Angels' fleet and both were cast into the Warp by the will of the Chaos Gods. Luther and his followers, corrupted by Chaos, became the Fallen Angels, while the Lion was returned to bowels of The Rock, the Dark Angels' fortress-monastery, where he slept for the next ten millennia.