Malenia is a Demigod of scarlet rot from Elden ring
She has a sword technique called waterfowl dance it's fast and deals a lot of damage(you should look it up cuz idk how to explain it). in her second form when doing this attack she makes ghost like versions of herself that also attack you.
She has a scarlet rot form and before she goes into this form she turns into a big rot flower that spreads rot/poison
She could be a demon with a blood demon art of rot or she could be a demon slayer with rot as a breathing technique
I think she also has a prosthetic arm with a sword that is made out of gold cuz her body is rotting or something. ( I think gold helps keep the rot from spreading but I'm not sure it's been a while since I played the game so something's might be wrong)