Upon activation, 1d4 characters in the party must make a successful Saving Throw, or be struck with total amnesia. Said characters will not remember who they are, how they got here, or who anyone else is. They won't remember their skills (including any magical spells), nor how to use any specialized equipment or devices.
What's worse, these characters will not be able to form any new memories either! Every 1d10 minutes, they'll forget everything, going through the same "Who are you people? How did I get here?" routine, over and over.
This effect will last until a successful Dispel Magic is performed, or until the characters are magically healed somehow, or until they exit the dungeon.
Optional: The effect continues after they leave the dungeon, and can only be removed through magic.