Abdemon was a notable Lieutenant Commander of the Emperor's Children Legion during the Great Crusade and the opening days of the Horus Heresy. His most notable action was during the Katara Xenocide, where he was charged with prosecuting a campaign against the Abhuman civilisation known as the Kartara. During the brutal campaign the Katarans sent diplomatic representatives under a flag of truce. They proposed to let the war be decided by a chosen champion from each side. At the end of the single combat the victor would claim not only victory for himself but for his civilisation.
Amazingly, the Lieutenant Commander agreed, and volunteered himself for the honour of representing the Imperial forces. He faced the Kataran champion in mortal combat and though wounded, was able to finally achieve ultimate victory. In an act that horrified the Emperor's Children, the leaders of the Katara immediately committed suicide and fell on their swords. The order given, their entire race followed suit and did the same. In honour of their courage and display of nobility, the Emperor's Children had the Kataran worlds turned into Mortuary Worlds, taking only their swords as relics in remembrance of the dead.
It is said that Abdemon and members of his Cohort trained and fought with the weapons of the Katara ever after, and bore these relics throughout the rest of the Great Crusade. Abdemon himself is believed to have died on the traitorous blade of one of his own captains on the field of the Choral City during the Istvaan III Atrocity.
The xenocide of the alien Katara was a brutal campaign of extermination carried out against this insular Abhuman civilisation that had evolved in the Kenuit System during the Age of Strife, turning irrevocably from the sacred purity of the human form and the fellowship of Mankind. When first discovered by an Emperor's Children Expeditionary Fleet, the first signs indicated that the star system's Imperial Compliance could be achieved without bloodshed. Initially offering no resistance, matters only deteriorated when the Katara were asked directly to formally surrender to the Imperium. Soon a lone Kataran warship arrived and destroyed the Light Cruiser Locrian before sustaining damage itself and withdrawing. So provoked, acting Lieutenant Commander Abdemon ordered the invasion of the largest Kataran city-complex the Imperial fleet's Augurs could detect.
Commencing an orbital bombardment which cracked open the upper levels of the Kataran city, the Emperor's Children landed, suspecting that they would face mass resistance to their invasion. Instead, they encountered lone Kataran warriors clad in strange armour made from an exotic metal who fought with leaf-bladed ceramic axes, plasma-tipped spears that burned like suns and swords of black glass that could cut ceramite with a delicate stroke. The skill with which the Katarans wielded their weapons was dazzling even to the Emperor's Children. The Katara's diplomatic representatives came forward again after hundreds of their warriors had died in their seemingly insane, staggered attacks and requested to meet once more under truce. The Kataran diplomat brought a proposal: let the war be decided by a chosen champion from each side. At the end the victor would claim not only victory for himself but for his civilisation.
Incredibly, Lieutenant Commander Abdemon agreed and said that he would enter the field as the Imperium's champion, while the Kataran Champion was known as the Hamaya. The two warriors fought, until at last both were covered in blood. That is when the Imperial champion got under the alien's guard and delivered the killing blow. With their champion defeated, the Kataran representatives ordered that all should know that their people had been defeated by an enemy of equal honour but superior arts. The order given, the Kataran leaders fell on their own swords before the Emperor's Children. Across all their domains every member of the Katara followed their leaders into death and the great Kataran cities became mass graves. Though some advocated stripping the now unoccupied worlds and beginning a process of resettlement, Lieutenant Commander Abdemon and the Emperor's Children refused, decreeing that the Kataran home planet would become a Mortuary World.
The cites would remain untouched until time remade them, and the corpses of their makers returned to dust. Relics were collected from their dead so that the Katara would be remembered: the body, weapons and armour of the Hamaya were set in a stasis coffer, and a token was taken from each of the Kataran warriors slain by the Emperor's Children. Throughout the rest of the Great Crusade, it is said that Abdemon and the members of his Cohort trained and fought with the weapons of the Katara. It is believed that Abdemon, who remained always loyal to the Emperor and the high ideals of the Imperium, fell to the traitorous blade of one of his own captains on the death fields of Istvaan III.
- The Horus Heresy - Book One: Betrayal by Alan Bligh, pg. 117