As I explained in an earlier video, the character of Dragon Slayer appeared as a villain earlier in the storyline, before the real nightmare began. In it, she was taken to Equestria while she experimented with magic and lost most of her memories of her own world.  Eventually she reclaimed her mind and set out to recreate her army in this dimension, forcing ponies to be overshadowed by her soldiers.

Eventually, she remembered Black Devil Girl, and how she was killed in a previous battle. Using her improved spells, she was able to recall the girl's soul and attempted to forcibly attach it to Fluttershy. However she was met with some resistence by Pinkamena Diane Pie and Rainbow Dash. Ultimately, Black Devil Girl was defeated in a battle with Pinkamena and Dragon Slayer was sent into an outrage. However after some thought, she realized how cruel she'd been and actually took back the souls she'd transplanted, returning the ponies to normal. She then used the legendary blade Excalibur to seal herself away in the form of Celestia and banished herself to an alternate timeline to rule in peace.

However this would not last and the desire to recreate her love took her yet again. She returned at a point where Equestria was in ruins (the current storyline) and decided to take advantage of its sorry state. She'd stand against Discord and vowed to strike him down, to claim Equestria for her own.

Because she cares so much for Black Devil Girl that it drove her mad and she would do anything she could to hear her voice and see her return, she chose "Anything For You" to represent her character.