As indicated by its composite name, Stygia-Aquilon is actually a binary planet comprised of two worlds tidally locked with one another. These twin worlds of fire and ice share the same orbit, their ellipse slowly taking them around a slowly-dying red giant sun. These twin worlds are best known as the homeworld of the notoriously bellicose Executioners Chapter of the Adeptus Astartes, which has called Stygia-Aquilon its home since the closing years of the 37th Millennium.
The Executioners' fortress-monastery -- the Darkenvault -- is no mere fortress erected in some desolate region of an already inhospitable planet, but an enormous armoured asteroid base trapped by gravity at one of the Lagrange points between the twin planets as they orbit around each other.
The Executioners are reticent to share much information about Stygia-Aquilon so that with exception of its location somewhere on the Imperium's fringes along the desolate void-wastes to the galactic south-southwest, between the borders of the Segmentum Tempestus and the Segmentum Pacificus, little is known of it.
In the wake of the Badab War and their part in it, the Executioners were condemned to undertake a hundred-standard-year-long Penitent Crusade during which they could not recruit any Aspirants to replace their losses. While currently unoccupied by the Executioners, the binary worlds have been entrusted in their absence to the care of the Salamanders Chapter and the Executioners' two Successor Chapters, the Iron Champions and the Crimson Axes.
- Imperial Armour Volume Two Second Edition - War Machines of the Adeptus Astartes, pg. 8
- Imperial Armour Volume Ten - The Badab War - Part Two, pp. 84-89