You've all probably figured out that I'm not much of a "Gotcha!" type DM. This is, of course, Tomb of Annihilation which is loosely based on the old Tomb of Horrors, which was the very height of Gygax's love for screwing the players no matter what they do. I love these characters a little too much to enjoy torturing them, but here we are.
Just blame it on Acererak. He's obviously a total bastard.
You'll also note that under normal circumstances, I hate to roll for the PCs. But I'm starting to learn that it's probably best if I do it in cases like this, where it would take quite a bit of back-and-forth and explaining to work out exactly what happened here (grabbing on where you can, rolling with the fall, etc etc - it was pretty complicated. I just rolled and worked it out. Mostly it worked out pretty well. Bad rolls could have made things MUCH worse.)
I hope my description was clear enough. From here, there's only a bit of time before Chrysagon's bracings fail. Before then Chrys needs to get out of the way, and if the others outside are going to join us, they need to get inside. (Personally, I don't mind losing a few NPCs here, easier for me to run without them!)
You might think I like having lots of NPCs around, and for RP purposes, I do (also, I tend to find it contrived that often no-one wants to help the PCs to get things done, even when it's important) but I also find them a pain sometimes. Overall, I'm happy either way.
Oh, and Imogen, Weed, and Dellrak should probably climb up into the next hallway. Better than going down!