The Nemendghast Raid was a reconnaissance in force carried out by an Ultramarines strike force against the forces of the Black Legion that had conquered the Industrial World of Nemendghast in the Vigilus System early in the War of Beasts.
These assets were to aid Abaddon the Despoiler in his invasion of the world of Vigilus in an attempt to close the Nachmund Gauntlet that provided the Imperium with the only known strategic passage through the Great Rift from the Imperium Sanctus to the Imperium Nihilus during the Era Indomitus.
At that time, Calgar despatched his Strike Force Shadowspear to determine why Nemendghast was no longer providing its requisition of supplies to the Imperial war effort on the world.
When the strike force discovered the sheer size of the Chaos forces present on Nemendghast, they moved from a reconnaissance in force towards a raiding stance and successfully destroyed the local Forge Infernus' ability to produce new daemon-possessed war machines and Chaos Space Marines.
But the price was high, and almost none of the Ultramarines survived the conflict.
Captain Sevastus Acheran took command of the Ultramarines 2nd Company from Cato Sicarius after that Ultramarines hero was declared lost in the Warp during his return to Macragge from Terra with the first Primaris Marines.
Acheran's first major operation was to lead a force of Vanguard Space Marines drawn from the 2nd Company designated "Strike Force Shadowspear" in a reconnaissance in force to the Industrial World of Nemendghast in the Vigilus System.
Nemendghast was a crucial supplier of war materiel to its neighboring world of Vigilus, which had come under assault by xenos and Chaos forces early in the Era Indomitus during the conflict remembered as the War of Beasts.
Chapter Master Marneus Calgar and the Ultramarines were now leading the defence of Vigilus, and Calgar despatched Strike Force Shadowspear aboard the Strike Cruiser Carpatia to investigate the curious silence of Nemendghast.
Upon arriving at Nemendghast, augur scans revealed that the planet's industry was not only intact but now producing Daemon Engines and the Possessed Chaos Space Marines called "Daemonkin" at a rapid rate.
It soon became apparent that the world had fallen to the control of the Black Legion in preparation for Abaddon the Despoiler's coming assault on Vigilus itself.
Knowing that the orbiting Black Legion warships would soon detect the Carpatia, Acheran ordered an immediate Drop Pod assault onto the planet's surface.
Upon seeing the Chaos forces that had now claimed the world, Acheran knew that Nemendghast could not be liberated with the forces he had on hand and instead he sought to foil the Black Legion's plans by destroying its daemonic forges.
Though vastly outnumbered by the Heretic Astartes forces, Acheran refused to withdraw and targeted the Forge Infernus of Golbrecht Hive. With the aid of the Librarian Maltis' psychic abilities, the Vanguard Marines were able to infiltrate the heart of Forge Infernus, bypassing the various Black Legion patrols.
Acheran then split his forces into multiple small, fast-moving detachments, assigning them to missions of sabotage and assassination. He prioritised the destruction of not only the infernal machinery used to draw on the power of the Warp, but also the assassination of the Warpsmiths who maintained and operated it.
Explosions soon rippled throughout the Forge Infernus before the Chaos forces even knew they were under assault. Desperately holding back the maddening tide of the Warp, Maltis realized that if the eight pillars of binding above the Forge Infernus' central spire used to draw daemons down into the hell forge could be destroyed, it would create a chain reaction that could bring down the entire complex.
Ulleus led squads of Vanguard Infiltrators into overlapping firing positions against Chaos Space Marine and Daemon Engine detachments that poured in to assail the Loyalist attackers. Ulleus and his warriors fought to the last, until a swarm of Venomcrawlers swept up from underneath to tear them limb from limb.
However this act of sacrifice bought Acheran the necessary time to launch separate attacks against each binding pillar. Each pillar was guarded by a pair of Obliterators, which Acheran countered with his Vanguard Suppressors. With the cover provided by the Suppressors' fire, Acheran led his Infiltrators forward, pushing past the Chaos defenders and setting the charges on each pillar.
The Black Legion forces on Nemendghast were commanded by the Master of Possession Vorash Soulflayer, who at first suspected that a rival warband of Heretic Astartes had launched a raid on his world. However when the truth became clear, he personally led a vicious counterattack.
The assault was led by hulking Greater Possessed and Venomcrawlers that tore through the ranks of the lightly-armoured Infiltrators. In orbit, the Carpatia revealed that a Chaos fleet had detected the Loyalist strike force's presence and was converging on the Ultramarines' position.
This force consisted of not only Black Legion Heretic Astartes, but also allied Chaos Space Marines from the Crimson Slaughter warband.
Knowing that his force's survival was likely now impossible in the face of such odds, Acheran decided to make a final defensive stand and sell his and his Astartes' lives dearly.
But first he ordered Maltis to escape aboard the Carpatia and return to Vigilus to reveal to Calgar that Abaddon's invasion of Vigilus was imminent.
Maltis successfully made his escape just as Acheran was able to destroy the last pillar, and the entire Forge Infernus collapsed upon both the Heretics and the remaining Loyalist Astartes.
- Shadowspear, Campaign Book, (8th Edition), pp. 6–9