Dragon Quest XI has already been mentioned but here's a clip in English of party member Jade (Martina in the Japanese version) turned into a demon and fighting her team:
https://www.youtube.com/watch?v=-MiSfp27l7U
Also does anyone know what doujin this picture below is from? It seems to be about HeartCatch Pretty Cure, with Cure Blossom and Cure Marine converted into loyal "little sisters" of a villain.