Area 22. The Cavern of Gold and Silver Mists
Past Area 21 the PC's go down a staircase to a long corridor. The corridor runs about 40 feet to a four-way intersection. There is a pit trap in the intersection. If the party keeps going east, they eventually come to Area 22.
Area 22 appears to be a natural cavern. It is filled with gold and silver mist. At the center of the cavern is a beautiful grotto. The whole place radiates a dim aura of good.
The mists are poisonous. Any PC who enters must save versus poison or have their intelligence drop to 2. Breathing "the clean air above ground under the warm sun." Will restore the afflicted PC's intelligence. So will a touch from the Siren who lives here.
See, as a private joke, Acererak has imprisoned a lovely Siren in this room. Here she is:
http://www.wizards.com/dnd/images/ToH_Gallery/ToHGraphic22.jpg
The Siren is friendly and will talk to the PC's. She cannot leave the room unless she is asked, but an enchantment keeps her from just coming out and saying that. Also, she doesn't know anything about the rest of the Tomb.
There are also two bags in the room. You can see them in the picture. The larger bag is actually a small bag of holding that contains a few coins. The smaller bag contains a random object anything from fluffy wool to a ring of feather falling.
Of course, there's a catch to all this. The PC's can take either the Siren or the large bag or the small bag out of the room. They can only take one of the three. If the PC's touch a bag, the Siren and the other bag disappear. If they ask the Siren to come with, both bags disappear.
IMO: On the one hand, I really like the concept of this room. It has an "enchanted fey" vibe that feels like a myth or a fairy tale. It's also a role-playing encounter, which is a nice change of pace from the rest of the Tomb.
On the other hand, the fey vibe seems a little out of place in the Tomb. Worse, there is no clue in the module to warn the players that they need to make a choice between the Siren, Bag A and Bag B. Because the players don't know the stakes, they can't make an informed decision. This means that they can't agonize too much about what to do. My guess is that most groups sort of blunder through this Area, accidentally pick up one of the bags, and then spend the rest of the night wondering what they missed.