The Battle of Diamat was a minor engagement of the Horus Heresy fought in 005.M31, shortly after the events of the Istvaan III Atrocity where the Warmaster Horus revealed his perfidy and openly declared war against the Emperor of Mankind. To ensure that the Traitors would be denied vital war materiel, the Dark Angels Legion deployed a small strike force to the Mechanicum Forge World of Diamat.
They were to secure heavy Ordinatii siege engines that were held by the Traitor forces. Though the Dark Angels would eventually emerge victorious and secure these powerful weapons of mass of destruction, they would unwittingly hand them over to a supposed "ally" -- the Iron Warriors of Perturabo, whose secret allegiance to the Warmaster was unknown at the time to the Dark Angels Primarch Lion El'Jonson.
When word reached the Dark Angels of the Istvaan III Atrocity, Lion El'Jonson and the I Legion were bogged down fighting a campaign against the Gordian League. Therefore, the Lion led a small war fleet of 16 vessels against the Forge World of Diamat, in order to seize Ordinatii heavy siege engines present there and deny them to the Traitors. He encountered some resistance from vessels in orbit of the world led by several Grand Cruisers loyal to the Warmaster's cause. After a fierce naval battle, the outnumbered Dark Angels prevailed with minimal losses thanks to the tactical abilities of their Primarch.
The Dark Angels then landed at Diamat's principle forge-city, sweeping aside what little resistance they encountered from Traitor Imperial Army forces and a Sons of Horus company stationed on the planet's surface. This battle was made more difficult by the betrayal of the Diamat's ruling Magos Archoi and his Skitarii Legions, who sided with the Traitors. In the wake of his victory, The Lion ordered the destruction of the planet's primary forges, which annihilated the remaining Traitor forces present on the world in a great conflagration and denied to Horus Diamat's manufacturing capabilities.
In the battle's aftermath, El'Jonson encountered the Legion fleet of the Iron Warriors, led by the Primarch Perturabo, who were on their way to Istvaan V to "pacify" the treacherous Warmaster. Unaware of their secret loyalty to Horus and his ambitions, the Lion unwittingly handed over the captured Ordinatii engines to be used against Horus, as he believed they would be more useful on Istvaan V than with the Dark Angels. This proved prescient, but not in the way The Lion had intended.
- Fallen Angels (Novel) by Mike Lee
|Battles of the Horus Heresy|
|004-006.M31||Battle of Prospero • Istvaan III Atrocity • War Within the Webway • Battle of the Somnus Citadel • Unrest on Caliban • Signus Campaign • Schism of Mars • Battle of Diamat • Drop Site Massacre • First Battle of Paramar • Battle of the Coronid Deeps|
|007-008.M31||Battle of Phall • Battle of Ravendelve • Battle of the Alaxxes Nebula • Siege of the Perfect Fortress • Chondax Campaign • Second Battle of Prospero • First Siege of Hydra Cordatus • Battle of the Furious Abyss • Battle of Calth • Battle of Armatura • Shadow Crusade • Percepton Campaign • Battle of Iydris • Thramas Crusade • Fall of Baztel III • Battle of Vannaheim • Second Battle of Paramar • Battle of Constanix II • Siege of Epsilon-Stranivar IX • Treachery at Port Maw • Manachean War • Mezoan Campaign • Battle of Bodt • Battle of Dwell • Battle of Molech|
|009-010.M31||Xana Incursion • Carnage of Morox • Sangraal Campaign • Liberation of Numinal • Battle of Arissak • Battle of Perditus • Battle of Sotha • Scouring of Gilden's Star • Battle of Nyrcon • Battle of Tallarn • Battle of Nocturne • Battle of Pluto|
|011-014.M31||Malagant Conflict • Battle of the Kalium Gate • Battle of Catallus • Battle of Tralsak • Tarren Suppression • Balthor Sigma Intervention • Scouring of the Ollanz Cluster • Battle of Zepath • Battle of Anuari • Battle of Pyrrhan • Second Battle of Davin • Battle of Trisolian • Battle of Yarant • Battle of Krade • Battle of Deluge • Battle of Heta-Gladius • Battle of the Aragna Chain • Battle of Kalleth • Battle of the Diavanos System • Battle of Desperation • Battle of Beta-Garmon • Defence of Ryza • Battle of Thagria • Solar War • Raid on Luna • Siege of Terra • Great Scouring|