Past the necrotic slime, the party comes to an opening leading to a single circular room some 50' in diameter. The floor of the room is a reflective, mirror-like black stone. The far wall is dominated by a large blood-red mirror.
Once a character sees their own reflection in the mirror out comes a Fetch.
A fetch is an undead reflection of the character. They have all the same abilities but appear as corpse versions of the characters. They fight as monsters with the same HD as the character's level, but only half their hit point total.
The mirrored floor has similar magic. When the fetch of the character is freed then a Shadow of the character also comes up from the floor. They are normal shadows, but neither they nor the fetches can be turned.
Whatever weapons the characters have their Fetch and their Shadow will have similar ones. Both monsters use the opposite hand of the characters. The Fetch appears as an undead version, the Shadow is a shadowed version.
Once both the Fetches and the Shadows are defeated they will crumble to nothing. No treasure.
The mirror on the wall will likewise crumble and reveal a passageway beyond.