Secret Ultimate Boss
To get access to a different final fight, you must make special preparations.
- Slavia must be in your party, but she can NOT be the main character
- You must obtain Obsidian Heart Pendant
After this, you must talk Sister Satan into wearing the pendant at which point, Slavia starts demanding that they torture her. If turned down, Slavia starts becoming more and more hostile until she attacks.
If you can defeat Nightmare Slavia, you get an achievement.