The Siege of Darkenvault was a valiant Imperial military campaign fought by the Executioners Space Marine Chapter in defence of their twin-homeworlds of Stygia-Aquilon against the predations of an unknown xenos species which nearly wiped out their Chapter during the closing decades of the 39th Millennium.
In what is now a largely forgotten episode of the Imperium's history, the southern fringe of the Segmentum Tempestus was assailed by wave after wave of Ork nomad fleets, xenoform migrations and strange psychic phenomenon which themselves caused a near epidemic of Witch Cults, alien infestations and daemonic incursions. Seen now by some sources within the Ordo Xenos as a precursor to the coming of the Tyranids, these events were largely unrecognised at the time as forming a pattern, as the eyes of the Imperium were focused on threats elsewhere.
Suffering severe attrition in a series of campaigns defending the Imperium's borders against invading Orks, Enslaver outbreaks and worse, the Executioners Chapter had been reduced to less than 300 Battle-Brothers when their own twin Chapter worlds of Stygia-Aquilon came under siege. The attackers were a nightmarish xenos species of worm-ridden, writhing charnel-feasters never before encountered by Mankind.
Again and again they struck without warning, their strange disk-like ships all but undetectable to the defences of the Executioners' fortress-monastery, Darkenvault, until the last moment of their attack when their ghostly energy rays seared through tens of metres of ferrous asteroid mass in an instant to spill a tide of horror into the fortress' lower levels. The resulting battles in the catacombs at the heart of the Executioners' fortress-monastery were unceasing and terrible, and in a short span of time few Space Marines remained to defend the inner sanctum and the Chapter stood on the brink of extinction.
It was the Astral Claws, supported by the Cruisers of Battlefleet Tempestus, that broke the siege of Darkenvault; their arrival came just as the last of the Executioners were gathering for a final stand. Many Astral Claws died in breaking the siege, and among their losses they counted their Chapter Master, Acas Seneca.
The handful of Executioners that endured owed a blood debt to the Astral Claws for the salvation of their Chapter, and swore that should the Astral Claws ever be so threatened they would respond in kind. It would be almost a century and a half before the Executioners returned again to full strength as a Chapter, and they would never forget their oath. When the time came over two thousand standard years later, they would heed Lufgt Huron's call, siding with the Tyrant of Badab and the Secessionists during the infamous Badab War.
- Imperial Armour Volume Ten - The Badab War - Part Two, pg. 88