Xi-Nu 73 was a Tech-adept of the 9th Maniple, Carthage Cohort of the Mechanicum's Legio Cybernetica, attached to the 1301st Expeditionary Fleet of the Word Bearers Space Marine Legion during the Great Crusade of the late 30th Millennium. This Imperial fleet was commanded by Seventh Captain Argel Tal of the XVII Legion's Serrated Suns Chapter.
Like many of his Mechanicum counterparts, Xi-Nu 73 had undergone extensive cybernetic enhancement, improving himself a great deal over two standard centuries of loyal service to the Imperium of Man. Carthage Cohort was comprised of dozens of maniples, of which Xu-Nu's four robotic wards were just one. The war machines and expert technical crews of Carthage Cohort were spread across the many Word Bearers fleets, rather than housed on their own Mechanicum vessels the way the Titan Legions were. Lorgar’s own insistence made it thus. The Carthage Primacy which gave this honour to the cohort had been signed and oathed over a century before the events of the Horus Heresy. The Carthage Cohort, one of the Legio Cybernetica’s proudest armies, would march with the XVII Legion, and dwell in the bowels of their vessels. Since that time, almost a dozen of Carthage Cohort's war machines had been inducted as honorary Legionaries of the Word Bearers.
Xi-Nu 73 had not been present at the time this ancient oath was sworn -- he had not even been flesh-born. But despite how useful the Carthage Cohort was to the Word Bearers Legion, the Astartes simply did not like the Mechanicum element in their midst. Relations were closer to cold than cordial, even taking the Mechanicum's inhumanity into consideration. Over time, Xi-Nu 73 -- who had most humbly risen to oversee a maniple of four robots within the Cohort -- came to realise that the Word Bearers were not like their Astartes brothers. It was an opinion shared by others of his rank, on those increasingly rare occasions he established contact with them. To Xi-Nu's perception, the principal problem was that the Word Bearers were too human; fuelled by emotion, rather than logic, which affected their tactical decisions and their very goals in the Great Crusade. Despite his personal misgivings, Xi-Nu 73 loyally served the Serrated Sun Chapter during many notable campaigns until the coming of the Heresy and the corruption of the XVII Legion by the Dark Gods.
Xi-Nu 73 died at the hands of the Imperial Custodes, when they attempted to torture the early Confessor Cyrene Valantion to find out the Word Bearers' plans at the start of the Drop Site Massacre. He attempted to save Cyrene by commanding his Conqueror-class robot Incarnadine to attack the Custodes, resulting in his death when they returned fire.
- First Heretic (Novel) by Aaron Dembski-Bowden, pp. 9, 43, 100-102, 104, 108, 114, 186, 187-193, 195, 203, 234, 236, 425-429