Where was I?
Area 18. False Crypt Protected by Fear Gas
As Bullgrit noted, the PC's are likely to bypass Area 17. It's set in the wall adjacent a down staircase. The stairs lead to a short hall, and then another set of descending stairs.
The hallway at the bottom of the second staircase is "slightly cloudy." Surely there's nothing suspicious about that! Turns out, there's fear gas down below there. Any PC who doesn't hold his breath has to make a save vs. poison or run off at top speed for 20 to 80 minutes. There's a door down here, but the fear gas makes it hard to find. There is "only 3 in 6 [chance] for any character to notice the south door."
Beyond the aforementioned south door is
Area 18A. The False Crypt, an elaborate hoax.
The door opens into yet another descending staircase. The stairs are filled with
Webs which can only be removed by magical fire. Any character who tries to break through them becomes "hopelessly entangled and can not get free unless magically burned free or wished out." Sucks for them.
At the very foot of the stairs is a silver-inlaid mace. If anybody picks it up, the mace starts to glow with a bright golden light. Also, there's this:
http://www.wizards.com/dnd/images/ToH_Gallery/ToHGraphic18.jpg
The PC's have entered a crypt (the module keeps calling it the "pseudo-crypt") filled with rotting and decayed furnishings. Among the furnishings are a jade coffer and an unrotted leather bag. A "lich-like" monster wearing a crown is resting on a sold-gold couch. It starts to get up when the PC's enter, and it is afraid of the silvery mace. A booming voice calls out, "WHO DARES TO DISTURB THE REST OF ACERERAK? IT IS YOUR DEATH YOU HAVE FOUND." The lich attacks!
Except it's not a lich. It's a special zombie with AC -4 and 32 hit points that attacks once per round for 1d8+2 points of damage. It can also absorb up to 12 levels of spells. It pretends to be casting spells, but really it just pounds on the PC's. If somebody hits the monster with the silver mace, it roars in pain and staggers back.
After three hits from the mace, the monster crumbles to dust, the mace shatters, and the room starts to collapse. The DM is encouraged to take time and carefully detail the rumbling, trembling and shaking that happens. Only the jade coffer, the leather bag, and the lich's crown are within easy reach. The DM should start counting slowly to 10, just as in Areas 2 and 16, and enjoy the show as the players panic and the PC's run up the stairs. The tunnel collapses behind them.
It's all an illusion. Nothing has collapsed. Everything is fine. The jade coffer contains six real healing potions. The crown is worth 25,000 g.p. The sack contains a few coins, a scroll of low-level spells and a fake treasure map.
IMO: At the outset, I've never known a group of PC's who would blithely walk into a "slightly cloudy" dungeon corridor. I would expect them to be prepared for poison gas. If they do fall prey to the gas, the PC's panic and run at top speed for 2-8 turns. As noted above, that's a long time. Where are they going to go?
The pseudo-crypt with the fake lich sort of presupposes that the PC's will pick up the silver mace, assume that the mace is useful against the fake lich, and use the mace to attack the fake lich. That's a lot to assume. Moreover, the silver mace is sitting at the foot of the stairs that lead down to Area 18a. There's no door between the stairs and the false crypt, and the room is pretty small. The PC's really should see the silver mace, the false crypt and the fake lich all at once. Finally, the whole thing ends with a big
Programmed Illusion of a cave in. How long does that last? If the PC's come back in two weeks does it still look like the Tomb has collapsed?
I'm not impressed with this room, but I'm not sure why. For one thing, it's a particularly poorly written area. The module's stream of consciousness style is very evident here. For another, the module presupposes a lot: that the PC's will take up the mace.; that the PC's will think the fake lich is Acererak; that the PC's will attack the fake lich instead of fleeing or negotiating; that the fake mace will strike the killing blow against the fake lich; that the PC's will fall for the illusory cave-in. It feels overly scripted to me.
Still, this isn't a death trap. The fake lich is not dangerous, does not cast spells and should be pretty easy to kill. The treasure is OK but no overwhelmingly valuable. Those facts are clues to the players that the module isn't over yet. My guess is that most groups are not fooled by Area 18.