Erud Vahn was a Techmarine of the Death Guard during the Great Crusade. One of the foremost "Forge Tyrants" of his Legion, he was a master of alchemic warfare and spent long years studying on Mars. When Mortarion declared for Horus at the start of the Horus Heresy, Vahn found himself more closely aligned with the Mechanicum and remained loyal. Vahn renounced his legion, becoming a Blackshield in a warband under Endryd Haar. Serving as Haar's second in command, Vahn fought in the Xana Incursion, where he was wounded by Haar after objecting to Haar's decision to help the Callidus Assassin raid the secret weapons facility beneath Xana-Tisiphone rather than simply capture the Ordinatus Ulator engines they had come for and withdraw. Haar spared his life because he was still needed, and Vahn reluctantly yielded. He deactivated the explosive collars on Xana's prisoners, allowing them to fight back against their captors, and went on to lead the force that captured the Ordinatus Ulator Ashurax.