In Fire Emblem: Three Houses, the secret underground facility in the Cindered Shadows DLC is called Abyss. Hidden beneath Garreg Mach Monastery, it shelters the Ashen Wolves, a secret fourth house whose members become central to the expansion’s story.
Byleth enters Abyss while pursuing an intruder and discovers that mercenaries are raiding the underground settlement. The Church of Seiros has maintained a concealed arrangement with the people living there, but some Church factions want Abyss destroyed rather than protected.
The DLC’s plot also introduces the Chalice of Beginnings, an artifact connected to an attempted resurrection ritual from the monastery’s early history. The underground setting is easy to confuse with locations such as the Holy Tomb or Shambhala: the Holy Tomb is tied to the Church’s sacred history, while Shambhala is the hidden city associated with Those Who Slither in the Dark. Abyss is specifically the Cindered Shadows refuge beneath Garreg Mach.