Fire Emblem: Three Houses — Cindered Shadows (Nintendo Cinematic Universe)

Fire Emblem: Three Houses — Cindered Shadows is a Japanese anime film.

Summary
A hidden fourth "house" is lurking below Garreg Mach Monastery - in a place known only as Abyss - in Byleth goes to investigate it.

Plot Elements

 * Byleth pursues an intruder into a hidden underground complex called Abyss, discovering a secret house called the Ashen Wolves; originally hidden through a secret truce, mercenaries have begun raiding Abyss.
 * Byleth and the students agree to help defend Abyss.
 * Aelfric, the Church's liaison to Abyss, warns Byleth that factions within the Church would like to see Abyss purged.
 * Aelfric theorizes that the mercenaries are looking for the Chalice of Beginnings, a key object in a ritual attempted during Garreg Mach's early days with the aim of resurrecting Sothis.
 * Aelfric is captured and ransomed for the Chalice, which Rhea reluctantly agrees to hand over believing the ritual lost. The Ashen Wolves also privately admit to Byleth that they are descendants of the four who performed the ritual, ordered by the Church to keep their bloodlines secret.
 * Aelfric is revealed to be the mastermind, capturing the Ashen Wolves and preparing to use the Chalice to resurrect Byleth's mother Sitri, for whom he held a secret love.
 * Byleth rescues the Ashen Wolves, and when he attempts the ritual anyway Aelfric is transformed into a monster and killed.
 * The Ashen Wolves are disbanded and allowed to leave Garreg Mach, with the four members to return to help each other and Byleth if needed.