You were the demon king, you fight the hero who is taking advantage in combat, but you had a beam up your sleeve, a spell to turn him into a girl, at one point you decided to cast it at him

But the hero managed to block it and throw it back all over your face

When the spell hit you there was a smoke that covered everything about you, it was different, your black hair had turned white and long, you had a girl's body, breasts, butt and curves

The hero confused by your new change would drop his sword and watch as you became a cute demon girl