Hydra Cordatus - Horus Heresy: Unknown to Captain Cassander, the Imperial Fists officer who had been left in command of the detachment of Space Marines that remained behind on Hydra Cordatus after the main fleet left, the Warmaster Horus had fallen to the corrupting influence of Chaos and had instigated a galaxy-wide rebellion in order to overthrow the Imperium of Man and raise himself up as the new Master of Mankind. Shortly after the disastrous events of the Drop Site Massacre of Istvaan V had played out to their tragic conclusion, the traitorous Iron Warriors Legion saw the opportunity to humble their hated rivals, the Imperial Fists, upon the isolated world of Hydra Cordatus. When the order came from Rogal Dorn for all of the Imperial Fists to return to Terra to begin overseeing the defence of the Imperial Palace, Captain Cassander had begun preparations to depart immediately, but the sudden death of his vessel's Navigator had left the detachments of the Imperial Fists upon Hydra Cordatus stranded until a replacement could be dispatched. The following day, word of the Warmaster's treachery and the massacre on Istvaan V reached Hydra Cordatus. Pride in an honourable assignment was replaced by frustration and bitter disappointment that the Imperial Fists present on the world could not fight alongside their Battle-Brothers, and could not call Horus to account for his perfidy and punish those who had trampled their oaths of loyalty to the Emperor into the dust. But the chance to make war on the traitorous allies of Horus came soon enough.The Iron Warriors made planetfall in the wake of a saturation bombardment that reduced the valley where the Cadmean Citadel was situated and the agri-settlements filling its fertile deltas to ash. Magma bombs and mass drivers boiled away the rivers and reduced fecund earth to arid dust. The Cadmean Citadel was left untouched, and Cassander still found it difficult to believe that such a precise bombardment was possible. But the Iron Warriors had purposely done this in order to show the Imperial Fists that they were superior to them in every way. The technological cunning of the ancient fortress builders, married to the artfully wrought geography and the courage of the defenders, proceeded to keep the Iron Warriors at bay for almost three months. Every day Cassander’s warriors stayed alive kept the enemy from redeploying and bringing their strength to bear elsewhere against the Loyalists. Yet, when the Iron Warriors finally overcame the citadel’s ancient defences and broke open its walls they would run amok. They would slaughter the remaining Imperial Fists Legionaries, the heroic men and women of Hyrdra Cordatus that had chosen to stand with them, and the refugees from the devastated fields below the fortress. Fifty-two Imperial Fists and thirteen thousand men, women and children were crammed within the citadel’s walls. When the final assault came, the Primarch Perturabo himself spearheaded an audacious attack upon the citadel's defenders, and slaughtered over thirty Imperial Fists Astartes himself in a span of only a few minutes. The rest of the Cadmean Citadel's defenders were slaughtered to a man and the surviving refugees were enslaved by the Iron Warriors before they moved on to their next objective. Hydra Cordatus was reduced to a barren desert world by the Traitor Legion's assault, and after the end of the Heresy was repopulated as an Adeptus Mechanicus research station and as a depository for the Imperial Fists Chapter's gene-seed.