Xenon's lair is the fortress that Xenon Karambit hid in before the Alliance defeated him and could safely be marked missing due to the ambiguous outcome of Act 40.
It is a citadel initially blending as a major building in what appears to be Studdton, a city in the Devil Beater universe.
The entrance to the lair resembles a regular medieval-style castle entrance, with cobblestone materials, styled architecture, and a fountain. It is relatively small.
Has guarded this area, following in the footsteps of Xenon's treachery and betraying Cielo after he was hired.
In these spacious L-shaped corridors, which contain nothing except for medieval-style windows and vent access, they serve as the last barrier between the Alliance and Xenon.
The last room in the citadel, it is a massive altar resembling a large church. With a purple theme and several pillars/arches reminiscent of 17th-18th century architecture, it crawls with Haze blight, as it appears to be in the vents, and is fuming out of two large pipes near the altar, the centerpiece. At the altar, there is a very large plaque with a yellow circle, and six colored lights. It appears they represent the seven elements (in order: Aether, Agni, Flux, Narukami, Helios, Stratus, and Haze), and might even be storages for the raw essence of the elements.
Xenon himself, of course, stood guard in the altar room before being eliminated. It is currently unclear on his current condition due to the ambiguous dialogue in the ending to his Arc.