The chapter appeared from the warp to the aid of a stricken Ultramarines vessel and fought off a chaos warband. They themselves had no understanding of how they came to be there, and once Inquisitor Battalion arrived - answering the Ultras distress call - he denounced them as heretics, despite the objections of the Ultras, and they had to flee, being badly damaged themselves.

Afterwards, the Iron Knights eked out an existance outside of Imperial law - lightning fast surgical strikes against small enemy targets (their presence has been noted in such campaigns as the Armageddon war, making seemingly insignificant, and yet with hindsight, critical attacks against smaller enemy support forces and minor reserves). They depend upon the kindness of sympathetic rescue-es for ammunition and occasional respite from Inquisitorial forces, though such is their desperation for such munitions that, tragically, small Imperial supply convoys are just as likely to come under attack from the knights themselves, as they are to be rescued from xenos and heretic attack by the Knights. This, coupled with the necessity that potential recruits must be cruelly abducted from their homeworlds, as well as Inquisitorial propaganda regarding the chapter means that the popular opinion of the chapter is, indeed, one of heresy, fear and suspicion.

The chapters pre-history ends with a cataclysmic battle for Morgue IV. A thousand armies congealed against the Imperium under the banner of Mors the Child. Imperial Guard, Inquisition and Adeptus Astartes forces descended upon the system in retaliation for Mors' incursion into the Imperial realm. The Iron Knights watched but could not join battle, for the risk of being set upon by would-be-friend and foe alike. Finally, as wars of attrition must, the tide gradually turned, and Mors prepared for a last stand at the capital and chief spaceport of the planet, Krux. The Iron Knights seized the opportunity and joined the fight, almost the entirety of the chapter, small though their numbers were, deep striking at the Ultra Marine vanguard of the Imperial assault, setting upon Mors and his personal retinue.

Conscious the promise of victory was ebbing ever further away, Mors retreated to his ship, the Mutiny, and prepared for a warp jump. Though fully aware of the fate their battle brothers - and chapter master - would be doomed to should they leave, the Iron Knights fleet pursued Mors into the warp, under heavy fire from the Mutiny as it did so. Neither hind nor hair has been seen of either force since.

Inquisitor Battalion, one of the Inquisitors present upon Morgue IV, at once denounced the Iron Knights. Not a word was uttered by the white-clad warriors as the Ordo-Hereticus paraded their 'basphemys' before the survivors of the battle, and arranged a hasty trial and execution of all Iron Knights on the planet. So rapid was the trial amid the confusion of the aftermath the Ultramarines, as well as the chapters ever-increasing number of supporters and sympathizers could not prevent it, yet they did appeal.

An Inquisitorial tribunal was called for, and Battalion was tried for crimes against the Emperor and a wavering faith, and was found wanting.