Megaman Zero 2/3.
Originally called the Mother Elf, she was created to end the Maverick Wars. Afterwards she was sealed away to prevent the misues of her power. But the evil Dr. Weil captured and corrupted her, turning her into the evil Dark Elf. This event sparked the Elf Wars.
The corruption was removed upon the destruction of Omega, Zero's original body.
"Before she was called the Dark Elf, she had another name..." ~Original X