In 935.M41, a daemonic horde led by M'kar the Reborn took control of the Indomitable, located in the outer orbit of Calth. Marneus Calgar, the Ultramarines' Chapter Master, commanding Terminators from the Ultramarine's 1st Company, boarded the star fort and (according to the official Chapter record) tore M'kar limb from limb .
However, Calgar was later forced to admit that he had deceived his Chapter. He revealed that he had defeated M'Kar, but, even with the 1st Company and the full might of the Ordo Malleus behind him, he was unable to destroy it completely. The best he could do was to imprison the daemon within the star fort's warp core. Half of the Ultramarines' Fifth Company was sent to garrison the fort, which was sent on a random course meant to lose it forever inside the Warp [2a].
In or around 854999, Warsmith Honsou of the Iron Warriors learned of M'Kar's whereabouts, and captured the fort in a bloody siege, slaughtering its garrison and freeing M'kar. The star fort became Honsou's temporary headquarters for his Invasion of Ultramar, and first appeared in the skies above Tarentus [2b].
By the time the Ultramarines responded, the fort was in orbit of Talassar, which had been all but destroyed. Calgar and the Ultramarines' 1st Company were shot down over Talassar and took shelter inside the ancient fortress of Castra Tanagra. In the weeks that followed, M'kar monitored the battle from inside the Indomitable, waiting until they were at their weakest before appearing in person to finish Calgar off [2c].
In the aftermath of M'kar's defeat by the Ultramarines, the Indomitable was destroyed by the Ultramarines Battle Barges Octavius and Severian, and its remains fell to Talassar and plunged into the ocean [2e].
Shortly before the Invasion, Honsou's bodyguard, The Newborn, had encountered a psyker on New Badab who predicted that his fate was "woven into the tapestry of a great hero's death, the fall of a star and the rise of an evil long thought dead." This memory was somehow communicated to Ventris, who in the aftermath of the invasion realized that the second part referred to the fall of the Indomitable.[2e]
- 1: Codex: Space Marines (5th Edition), pg. 40
- 2: The Chapter's Due (Novel) by Graham McNeill
- 3: The Skull Harvest (Short Story) by Graham McNeill