The Endurance was a Space Marine Battle-Barge that served as the flagship of Mortarion, the Primarch of the Death Guard Legion during the Great Crusade and the Horus Heresy in the late 30th and early 31st Millennia. This flagship rode at the head of the flotilla of warships that comprised the XIV Legion's Expeditionary Fleet. The mighty vessel, like all the starships within the Death Guard Legion fleet, were cut from steel the shade of a stormy sky, with a prow sheathed in dark ocean green. It moved as a slow dagger might in the hand of a patient killer, inescapable, inexorable. It bore little in the way of ornament. The ship’s only decorations were martial in nature, etchings on the plough-blade bow in letters the height of a man, long lines of text that recalled an age of battles fought, worlds visited, opponent’s lain to wreckage. Her only adornments of any note were two-fold: a golden spread eagle with two heads across the face of the flying bridge and a great icon made of heavy nickel-iron ore, a single stone skull set inside a hollow steel ring in the shape of a star, at the very lip of the spiked blade, watchful and threatening. Her escort ships would fall into line behind her, taking up a formation that mirrored the spear tip battle-patterns of the warriors that were her payload. In echo of the unbreakable resolve of those fighters, the warship proudly bore a name in High Gothic script across her iron hull: Endurance.
The Endurance is known to have transported the Death Guard contingent to the Triumph of Ullanor, where Horus was elevated to the esteemed rank of Warmaster by the Emperor Himself in recognition of his numerous great deeds and outstanding leadership throughout the Great Crusade. This flagship also transported the Death Guard to the historic Council of Nikaea, to determine once and for all whether the use of psychic abilities within the Space Marine Legions should be allowed. The Endurance was present during many of the notorious events that occurred during the Horus Heresy which included the Istvaan Campaign. The Endurance was one of many ships of the Traitor Legions present in the Istvaan System that willfully turned upon their former brethren and virus-bombed the planet of Istvaan III on orders from the Warmaster, in order to wipe out Loyalist elements of the various Legions under Horus' command. The Endurance was also present during the infamous Drop Site Massacre at Istvaan V that saw the Iron Hands, Raven Guard and Salamanders Legions nearly wiped out, and rendered ineffective as fighting forces during the rest of the Horus Heresy.
During the latter part of the Heresy the warriors of the Death Guard found themselves becalmed in the Warp and assailed by Warp-born plagues so virulent that not even their legendary resilience could withstand them. Soon, the entire XIV Legion was beset by a sickness that bloated their bellies with corpse gas, caused flesh to slough from their bodies and made these strongest and toughest of warriors into crippled wretches assailed by delirium. Though none can say exactly what forces acted upon the soul of the Primarch of the Death Guard, whether he was already damned or whether he made his pact with Chaos in some state of fever, he must have called out for deliverance, and his call must have been answered. From the stygian depths of the Warp the great entity that was Nurgle, Lord of Decay, Father of Disease, opened his arms and embraced Mortarion and the Death Guard Legion, drawing them to his breast and making them his own. When finally the Death Guard Legion’s fleet emerged from the Warp during the Battle of Terra, its vessels and its warriors were entirely changed. The once-gleaming white and grey armour's heraldry was stained with filth and a sickly pall of greenish hue. The once-noble warriors were transformed into walking hives of death and abomination. Condemned to a deathless state of decay, the Death Guard would spread their pestilent diseases the length and breadth of the galaxy for the greater glory of Chaos.
With the ending of the Horus Heresy, the Primarch Mortarion led his Legion into the Eye of Terror, and while the other Traitor Legions had splintered into countless warbands, the Death Guard remained largely whole, thanks in no small part to their legendary strength and resilience. Mortarion led them to a world that would become known simply as the Plague Planet, which he moulded into a new and despicable form, making it a virtual copy of the Legion's homeworld of Barbarus. To this day, Mortarion’s Death Guard launch their assaults through the Cadian Gate and into the galaxy beyond, sometimes in large bodies and at others lending strength to allied Chaotic forces. What final fate befell the Endurance, or whether it still remains the flagship of the Daemon Primarch Mortarion, is unknown.
One of the few notable sections of the Death Guard flagship was the Assembly Hall, which in itself was an unremarkable space. It was nothing more than a void in the Endurance’s forward hull, rectangular in aspect, open at the far end to the stars through two oval panes of armoured glass holding out the killing vacuum. There were louvered shutters half-closed across the windows, casting patterns of dim white light in bars where the glow from a nearby nebula reached the vessel. The ceiling was an arch, formed from the primary spars of the warship’s iron ribcage where they met and meshed in steel-riveted plate. There were no chairs or places where one might rest. There was no use for them. This was not a hall in which lengthy debate and plots would be hatched, but a place where blunt orders would be given, directives made and battle plans drawn in swift order. The only adornments were a few combat banners hanging down from the metal beams.
The room was littered with shadows. Alcoves formed from the spaces between the girder ribs went deep and ink-black. Illumination would fall in pools, tuned to the same yellow-white of high sun on Barbarus. In the centre of the chamber, a hololithic tank turned on a lazy axis, a ghostly cube of blue drifting there. Mechanicus Tech-adepts would often be found working around the disc-shaped projector device below it, moving in orbits around each other, but never straying more than a hand’s length away. Although Endurance was not large enough to hold the entire Legion, many of the Death Guard’s seven Great Companies were often represented, in whole or in part.
The only other notable feature of the Endurance was its Commandery, the nexus of private chambers and sanctorum the XIV Legion's Primarch took as his own while he was aboard. Like all Death Guard, the Primarch kept the commandery just as austere and functional as the rest of the vessels under his command. There was only a single large chart table and chair where battle plans could be laid out and studied. When the Primarch sat there, his armoured form would be framed by a tall stained-glass window that looked out over the length of the warship’s dorsal hull.
High up atop the Endurance’s dorsal hull lay the oval dome of the ship’s Observatorium, a space put aside so that the vessel's naval crew might be able to take emergency star fix sightings should the vessel’s Cogitators become inoperative. It also served a purely ornamental function, although there were few among the Death Guard who ever used it for so trivial a purpose.
- Flight of the Eisenstein (Novel) by James Swallow, pp. 5-6, 13, 60, 90, 115