Event · M31

Battle of Diamat

The Battle of Diamat was an early Horus Heresy action in which Lion El’Jonson led a small Dark Angels force to the forge world Diamat to deny its heavy siege engines to the Warmaster Horus. Word of the Isstvan III atrocity had reached the Lion while the bulk of the I Legion was still engaged against the Gordian League in the Shield Worlds, a commitment Horus had helped to arrange. Able to spare only about sixteen warships and several thousand legionaries, the Lion chose Diamat as the point at which a limited strike could still shape the coming war for Terra. On arrival the Dark Angels faced a larger Traitor Imperial Army fleet. They prevailed in the void through the Lion’s direction, then landed at Diamat’s principal forge-city and swept aside the first resistance. A counter-attack by Sons of Horus, joined by the defection of the ruling tech-priest Archoi and his skitarii, nearly reversed the gain; the Lion turned the battle by destroying the primary forges as they were overrun, annihilating the attackers and denying Horus the world’s manufacturing capacity. The siege engines already secured remained in Dark Angels hands. In the aftermath Perturabo arrived with the Iron Warriors, outwardly loyal and bound for Isstvan V. The Lion, seeking later support for a claim to the office of Warmaster once Horus fell, transferred the engines to Perturabo. The Lord of Iron was already secretly sworn to Horus, and the engines went over to the Traitors. Diamat was a tactical success that became a strategic gift to the rebellion.

See where Battle of Diamat occurred on the galactic map during the Horus Heresy.

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