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Mordius was one of the first human warlocks that ever existed. He became to study it during the Second War and became deeply interested in summoning demons and unleashing doom on opponents. He used this in battle for the Alliance, but they told him that demonic magic was forbidden in the Alliance, and he was not allowed to use it in combat.
Mordius did not give up the warlock magic, he continued to study it for himself. He became very talented over time, but one day during the Third War the Alliance arrested him, thinking he was in league with the Burning Legion. He protested and told them that he would never join the Legion's ranks, but it did not help. He was sentenced to prison, where he sat until the day when Jaina Proudmoore was about to sail over to Kalimdor, following Medivh's instructions. Jaina asked him to come with her, due to that he was extremely skilled in demonic magic and she needed his help with the Legion's use of magic. Mordius accepted, but before they managed to get to the ship that was to carry them, Mordius was slain. Before he died, he thought that he died a brave death at least, but it wasn't over yet. Mordius was risen as an undead and became one of the Lich King's slaves.