the Jester said:
Whoa- I've neither seen nor heard of this before!
What is it, if I dare ask?
The article, written by Bruce Cordell as a companion piece for RttToH, deals with (as you may have guessed) what's below the Tomb. The rationale here is that Acererak had a lot of servants and minions constructing his Tomb, and when the work was done, he murdered them all and sealed them in the catacombs below the place (he'd had them excavate some of those catacombs beforehand, always planning to bury them there when they were done). The hook for PCs is that only one person ever learned of the Undertomb and survived, having gotten out with the little bit of information left in his shattered mind scribbled on a map - the only way into the Undertomb is by pulling up the stone that makes up the floor in the Chapel area in the Tomb itself.
However, many of those workers still reside below the Tomb as undead, due to the latent negative energy Acererak unleashed in his many schemes. Worse yet, the master architect remains here, believing he was imprisoned because the traps he designed for the Tomb didn't please his master, and has spent his centuries designing more and more deathtraps, hoping to draw Acererak's approval.
The article doesn't feature a keyed map, but instead lists just a few of the deathtraps likely to be met down there. After that is a discussion about the residents of the Undertomb, and a section about their leader, Moghadam. Following that is a section on the potent magical items and spells used in conjunction with the Undertomb. Needless to say, this is a really great article.