At the first opportunity, I wanted to find out what would happen if I captured the red castle without completing the other quests. However, I only learned two things:
1. The castle is inhabited by the owner of the Artifact of Might, the Amulet of Mastery, and a huge army. It's scary.
2. I need to urgently replace the "banishment" ability with something useful. In version 3.0, banishment could only banish one archdevil. It will be difficult for Zehir in the final mission of the original campaign.