Iddinam was a notable Lord Commander of the Emperor's Children Legion during the Great Crusade and the Horus Heresy eras. His most notable action was the Praxil System Compliance, where he was the acting commander of a sizable Emperor's Children force charged with bringing this newly discovered system into Imperial Compliance. Despite his best efforts, the campaign did not begin well, and there were those amongst the Imperial forces who feared that Iddinam would be unable to bring the system into Imperial Compliance without help. Sending a request to Terra for reinforcements, soon elements from the Blood Angels and the Imperial Fists Legions arrived, much to the chagrin of Iddinam. Reluctantly, Iddinam put these supporting assets to good use and was able to finally bring the recalcitrant star system into Compliance, but the sting to his pride grated on the Lord Commander, who refused to be present at the Compliance ceremonies at the successful conclusion of the campaign. This would prove a fateful decision, as Iddinam would later have a hand as a prime conspirator of his Legion's treachery during the Istvaan III Atrocity.
Lord Commander Iddinam was the commander of an Army Group of Emperor's Children charged with bringing the Praxil System into Imperial Compliance in order to acquire its vast wealth and natural resources without also damaging its delicate infrastructure. The Emperor's Children were supported by selected elements of the Imperial Army and the military forces of the Adeptus Mechanicus. Despite meticulous planning, the campaign did not begin well, as substantial losses were inflicted upon the Imperial forces. Changing their strategy, the Emperor's Children decided that instead of focusing on systematic conquest of the system's void stations, moons and planets, they would instead focus upon Praxil's fleet. Without the ability to redeploy their strength, each of the Praxil domains would become a closed tactical problem. But the people of Praxil had anticipated such tactics, and instead deployed multiple strike fleets to attack the Imperial fleet.
Further heavy losses were incurred during this initial engagement as the Praxil took advantage and attacked the Imperial toehold on their outer moons and space stations. A representative of the War Council of Terra observed that Lord Commander Iddinam would never willingly admit that he needed assistance, and therefore issued a request to Terra for reinforcements. Soon elements from the Blood Angels under the command of Chapter Master Raldoron and Marshal Cazimus of the Imperial Fists arrived in-system to augment the beleaguered Imperial forces. Despite his misgivings and the slight to his personal honour, the Lord Commander accepted his cousin Legion's; assistance and immediately drew up plans for prosecuting the second offensive against the Praxil. Utilising the strength of the Blood Angels and Imperial Fists, the Emperor's Children launched a successful second offensive against the Praxil that soon saw an Imperial victory. When the Imperium claimed its triumph, however, the Lord Commander refused to be present during the subsequent Imperial Compliance ceremonies or acknowledge his role in effecting the Compliance of Praxil afterwards. Iddinam would later have a hand as a prime conspirator of his IIIrd Legion's treachery during the Istvaan III Atrocity. He gave himself over to the horrors of the Warp before finally being destroyed by the Loyalist Iron Warriors Warsmith Auric Saxton at the Battle of the Harrow Ravening.
- The Horus Heresy - Book One: Betrayal, by Alan Bligh, pp. 118-119