In one WIP, I have a character becoming so corrupted by power that she turns into a demon-like creature. It happens right after she kills her sister to "win a game." I originally thought that she just became so corrupted that the act of killing a loved one twists her into something dark and unnatural. But I'm worried that's not a good enough explanation. Maybe she should have to drink some concoction? Or go through some ritual? Or maybe just leave it as is? Any suggestions?