The Holy Tomb is a site sacred to the Church of Seiros. It is a large, underground chamber housing caskets that contain Crest Stones, as well as a throne overseeing the expanse.
During a New Game+, it is possible to unlock the ability to visit the Holy Tomb outside of battle by visiting the amiibo Gazebo and scanning a Fire Emblem amiibo. During such visits, the player can wander the space surrounding the throne and find rare item and ingredient spawns. While it is possible to look into the distance at the wider expanse of the tomb where the battle in Chapter 11 takes place, the player cannot descend the stairs to explore. Depending on when in the game the player visits the Holy Tomb, they will either encounter Sothis or the opposite gender version of Byleth.