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