In it, a group tome of horrors 3.5 pdf adventurers travel to the titular tomb to fight the demilich Acererak and recover any loot found along the way. Anyone who has ever played this adventure has had characters die horribly in the Tomb. The tomb is filled with dozens of deathtraps.
Out of print for many years, it has recently been made available again as a PDF. 5 ruleset of the original super-lethal module, the official update is actually a standard, mostly-balanced dungeon crawl, aimed for level 9 characters. There are two versions in Fourth Edition. One is merely a conversion of the old tomb. Acererak works to ascend to godhood and beyond. While not as lethal as the original, owing to rule changes, it’s still quite deadly, and the Acererak at the end is a proper demilich again with all the terrifying powers you’d expect him to have.
There is an earlier encounter that some players will think is Acererak but isn’t. And later games and versions reveal that what the party fights at the end of the first game was a Demilich construct, not the real one. The writers planned for ethereal travel, melding into stone, magical defenses, teleportation, etc. Even if you kill the Tortured Vestige, it reforms a day later as long as Moil exists. There is a hallway filled with mist that reverses gender and alignment when you pass through.