Remixing Avernus - Part 4A: The Road to Candlekeep
In order for Descent Into Avernus to continue beyond this point, the PCs need to go to Candlekeep and have Sylvira Salkiras open the infernal puzzlebox. (Theoretically they could also figure out an alternative way of opening the puzzlebox and also an alternative method of getting to Hellturel after they do so.