Juin no Onna Kenshi @COMIC

This was a time when mages were feared. In the Kingdom of Fulgund a female spellsword called Alfiris travels hoping to be "a person who can save others." However, those who manipulate magic are shunned even by the mercenary guild. If any travel companion would see her "cursed brand"...

One day she is reunited with Sister Anorn, a drunk who considers her a "friend," who is gathering a party to slay the Demon Lord. But when overwhelming force strikes them, Alfiris's right arm bursts into flames and a different persona emerges in the face of despair!

(Source: NicoNico Seiga, translated)