Abyss is an underground area in Fire Emblem: Three Houses and primarily appears in the Cindered Shadow DLC side story.
Deep underground below Garreg Mach Monastery lies a series of secret tunnels that lead to an entire underground town known simply as Abyss. It is here lies various vagabonds who find solace in the depths. Their reasons for coming to Abyss varies; according to Balthus, they could be shunned nobility, merchants or simply looking for some peace.
Abyssians keep themselves safe from the surface world by sealing off entrances to the underground, though this has not stopped rumors circulating amongst the people of Fódlan. The Church of Seiros has maintained a treaty with the Abyssians - they will not interfere with their affairs, as long as the Abyssians do not interfere in theirs.
One Cardinal of the Church, Aelfric, serves as a bridge between the Abyss and the world above and desires to help them as best as he can.
Among its denizens are several Academy age individuals, some of whom are former Officers Academy students. At Aelfric's insistence, a fourth house, the Ashen Wolves, resides in Abyss, though it is a house in name only with no formal or official recognition by the Officers Academy.
Merchants of the Abyss offer many of the same services as merchants found in the Garreg Mach Monastery marketplace. Battalions can be replenished, weapons can be repaired or forged, and items can be found for sale. Not all citizens of Abyss trust merchants that sell their wares there, however, as many merchants have dealt in stolen goods.
Abyss is also home to the Shadow Library. Texts in this library include banned, obscure, or otherwise lost books that cannot be found in Garreg Mach's own collection and detail a hidden side of Fódlan's history.