fey-uplifted anthropomorphic corgi bard (with a 20 charisma!!!). Near the end of the adventure, they were joined by a cleric tour-guide from one of the local cults. This was a HUGE group for me (I usually run between 3 and 5) but things ran smoothly and most of the combats moved along at a good clip.
Spoilers for the adventure follow, so you should probably stop here before reading further if that's a thing for you.
B4 was written by Tom Moldvay who also authored X2: Castle Amber. Where X2 is unabashedly inspired by the writing of Clark Ashton Smith, B4 is a bit more subtle in being influenced by Robert E. Howard’s Red Nails.
The default hook for B4 is that the PCs were part of a caravan crossing a desert, got separated from the rest in a sandstorm, and stumble across the pyramid while searching for food and water. This hook works poorly if there are any clerics in the group (who can create up to 20 gallons of water each per day) and utterly falls apart if there’s a druid who can cast goodberry. 5e is most definitely not a game about logistics.
You can, of course, use a different hook, but the starving one works so well to give the PCs a sense of urgency that it’s a shame when you can’t use it. Of course, if the PCs’ gods want them to go into the pyramid…
Most of the pyramid is built around the kinda-sorta four-way battle between four cults. Gorm, god of justice (who only accepts male fighters as full members), Madarua, goddess of birth, death, and changing seasons (who only accepts female fighters as full members) and Usamigaras, god of healing, messengers, and thieves (who favors spell-slingers but is pretty much accepting of everyone) are kinda-sorta aligned against relative newcomer Zargon, a tentacle horror worshipped as a god who lives at the lowest level of the pyramid. However, the three cults of the old gods spend most of their time fighting each other, leaving Zargon to rule the local roost.
This conflict is the best part of the adventure. It’s relatively easy for the PCs to join one of the original three cults (and encouraged, since the cults are really the only way for the PCs to resupply during this adventure). Usamigaras’ representative in the pyramid, “stout Auriga Sirkinos,” makes an excellent villain for the upper levels, especially since the first group the PCs will almost certainly meet will be the followers of Gorm.
(Technically, the PCs could join the cult of Zargon, but this is discouraged by the adventure. Still, this could take the game in some interesting directions and would work well for an evil party or one that’s built around infiltrating their foes and destroying them from the inside. Also, it’s not assumed the PCs will all join the same cult, though some of them joining the cult of Zargon will almost certainly result in the party being split, and create all sorts of headaches for the DM.)
Unfortunately, the way the upper-most levels are designed, the players can be excused for thinking they’re in some sort of random, fun-house dungeon. They could spend the entire first session fighting fire beetles, stirges, sprites (who flee out a cartoon-esque tiny door near the ceiling if pressed hard), and giant centipedes before they even discover that the place is inhabited by humans. By the time they encounter the giant bees guarding the treasury of the Brotherhood of Gorm, they’re likely to think that giant bees guarding a treasure is just par for the random-dungeon course.
So if you do run this, I’d play up the inhabited feel of the place: the way the statues on the roof still function and have been recently oiled, the resetting of the traps, things like that. Encourage the mystery of this seemingly lost ruin being inhabited.
Unfortunately, the PCs can get involved in the conflict by the time they’ve reached the 10th room in the key. Yeah, I know, I’m being inconsistent here: first I complain that it takes too long to find the interesting inter-cult conflict, and now I complain it comes too soon. Here’s my issue: by the time the PCs get involved in the cults, they have much more entertaining things to do than explore the other 90-or-so keyed locations in the pyramid. Sure, you could send the PCs on various quests inside the pyramid on behalf of their patron cult(s), but there’s more fun to be had in the underground city beneath the pyramid.
This weekend’s game did just that: presented with priests of Zargon who claimed that the PCs were the answer to ancient prophecy, they willingly followed them to the promised hidden city because hidden underground cities sound cool. Only the top two tiers of the pyramid were explored. The rest of the time they spent liberating treasures from the mausoleums on the Island of the Dead. That part is only suggested by a few lines near the back of the book and a broad map of the underground city that’s very much lacking in detail.
If you’re going to run this one, don’t spend too much time pouring over the lower levels of the pyramid, but do spend some time working out the details for the four factions. Do figure out what they might want from the PCs and how they’ll react to the chaos the PCs wreak. Do spend some time working out the details of the underground city. If your players are at all the sort to chat with the people they come across and not attack everything that moves, they’re likely to become a lot more interested in the Lost City and its secrets than they are in delving the pyramid’s decidedly deadly dungeon.