The Marines Mordant was a Loyalist Space Marine Chapter of unknown Founding and a Successor Chapter of the Ultramarines raised in the 35th Millenium. Its Chapter planet is the Mining World of Vitrea Mundi located in the Absinthia System. The Chapter was obligated to undergo a one-hundred-year penitent Crusade through the Tempest Hippocrene in recompense for some unknown offence or dishonour. In the last years of the 41st Millennium, the Marines Mordant lost two companies in battle with the forces of Hive Fleet Leviathan, as well as suffereing severe damage to its Battle Barge Assiduous.
The Marines Mordant had been tasked alongside their fellow Ultramarines Successor Chapters the Crimson Consuls, Vindicators, and the Nova Legion with guarding the rimward frontiers of the Maelstrom Zone. However, in late 41st Millennium, all four Chapters were destroyed in sudden systematic attacks on their Chapter homeworlds, through the machinations and using the forces of the Alpha Legion Chaos Lord Quetzel Carthach.
- Assiduous (Battle Barge) - The Assiduous took severe damage in battle with the Tyranids of Hive Fleet Leviathan.
The Marines Mordant's Chapter colours are blue and silver.
The Marine Mordant's Chapter badge is not listed in current Imperial records.
- Legion of the Damned (Novel) by Rob Sanders, pp. 26, 97
- Sons of the Hydra (Novel) by Rob Sanders, " α The Serpent's Egg," "β Spitting Venom," "γ Turning Tail"