Frieren: Beyond Journey’s End: 1x10

A Powerful Mage

Mortally wounded, Lügner is left to wonder how Fern is so powerful, until he comes to the realization that she has been suppressing her mana to look weaker than she actually is, which is considered taboo among demon mages. Fern then finishes off Lügner. In a flashback to a thousand years ago, Flamme comes across a destroyed elf village and finds Frieren the only survivor, having managed to kill the demon general attacking her. Flamme decides to take Frieren on as her apprentice and teaches her how to indefinitely suppress her mana.