Just a quick thought regarding quicksand from the ankhrav encounter in the last section. It seemed weird that as soon as players stepped into quicksand they immediately fell in up to their middle and then on a good initiative roll, the quicksand could absorb them up to their neck before they get a chance to respond.
That feels unfairly dangerous, especially given the difficulty of escape and the awful consequences of going under and suffering from the drowning rules--which are just brutal.
I'd argue that a starting condition of immobilized would be a better start. The character steps into the muck and it swallows them up to their ankles and they realize with horror they can't free their feet. Roll initiative. If the quicksand wins, they get pulled down to their waist. If not they have a chance to fall back onto land and free themselves.
That feels more like quicksand, both the way you see it portrayed in the media as well as how it works in the real world.
This happened to me and 2 other players at my table last night.
We wen't able to react or attempt to really do anything until we were at our necks in sand.
This happened to the lead PC in our party, but I could't see any reason to have the reaction affect multiple PCs. That seems a bit brutal, but it did make for a more interesting encounter.