Xana II is a Forge World of the Dark Mechanicus within the Eye of Terror, which keeps popping up in the Imperial Armour books about the Siege of Vraks, and in "War Machines of the Lost and the Damned".
The oldest reference to the mighty Hell-Forge of Xana II appears in the introduction to the final volume of the writings of the cursed Heretic archivist of the Gethsemane Reclusium, purporting to catalog certain terrors that lurk within the Eye of Terror beyond the ken or mortal savant and beyond the reach of even the keenest of assassin's blades. Xana II seems to be the source of several classes of Attack Craft frequently utilized in the armies of the Great Enemy, the most notorious of which are the Hell Blade and the Hell Talon. Without exception, all of the abominations produced by this Hell-Forge are baleful aberrations, taints upon the fabric of reality. These unholy creations fuse man, machine and deamon in ways never seen before, often being so repellent to realspace that they cause visual and auditory hallucinations as well as atmospheric disturbances. Along with the stables of the Dark Mechanicum like Reaper Autocannons or Havoc Launchers, this forge-world also produces eldritch weaponry such as pyrax incendiary bombs, alchem cluster bombs, or even fell warp-pulse bombs which slay enemies in ways repulsive and disturbing to even the most jaded servant of the Corpse-Emperor.
While Xana II itself is located within the baleful energies of the Eye of Terror, craft matching the description of the Hell Blade and Hell Talon have been sighted in war zones across the galaxy, suggesting that its masters -- themselves known to be singular hybrids of man, machine and Daemon -- have established bonds with factions across the void and are supplying them with weapons and materiel to further their own ends. To date, the greatest concentration of attack craft Imperial forces have faced was at the infamous Siege of Vraks, but recent confessions extracted from captured Hereteks suggest a mustering of unprecedented scale beyond the Cadian Gate for the 13th Black Crusade.
As of the Horus Heresy book 6, the original Magos who lead Xana II in the name of the warmaster has rules, and statistically beats said warmaster and any other Primarch in a challange.
Of course this name is a reference to the ever-popular to include in your science fiction glossary term "Xanadu" - as this is derived from the name of Kublai Khan's summer capital, one wonders why they blew that load here and not all over the White Scars, but it is what it is.