The Flame Emperor Army is a militant faction led by the Flame Emperor and their lieutenant, the Death Knight in Fire Emblem: Three Houses. The objectives of the Flame Emperor Army are unknown, but it seemingly opposes the Church of Seiros and is also working at odds with Those who Slither in the Dark.
In Imperial Year 1180, the Flame Emperor sent Kostas and his band to attack Edelgard, Dimitri, and Claude, three students of Garreg Mach Monastery's Officer's Academy who were the heirs of their respective countries, while they were on a camping trip. The students were saved thanks to the timely intervention of Byleth and Jeralt, and Byleth was revealed to have a mysterious power: the Crest of Flames. The Flame Emperor was intrigued by this development and decided to monitor Byleth's activities.