Not necessarily a suggestion, just some ideas I put forward to think over.
This whole descending in this dungeon can just be a "stage" I believe you had allready established in a conversation with the witch that space has been messed with, for all we and the characters know we are at the other end of the continent or even in another dimension.
There are also multiple parties involved in the dungeon one of them might be able to and choose to move/close the entrance of the dungeon thus requiring the party to:
-find the next entrance on their own. -Wait till it opens. -Force it open. -Report back to their superiours and make use of their organisations to find the next entrance.
This would also give you space to work with regarding lore and worldbuilding and giving you another way to showcase their corruption. While also giving you another avenue to expand the game beyond the dungeon.
Another example would be that their organisation would deem a/their knights to be unfit to continue this quest before they have tried to "cleanse" them because they seemed to visibly corrupt.
Potentially switching out a team member for a mission or having to do a mission with 1 less.
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Also the stairs back to camp could perhaps be closed off/moved necessitating the party to sleep/camp inside the dungeon with all the issues that come with it.
You would thus also be leaving the player character outside alone.....
Anyway I will stop here just wanted to throw some ideas out there.
As of now I haven't played the new update yet so I will get to that.
Good luck