The PCs were not in trap-finding mode, so ate the 14d6 fireball, losing the wizard instantly. The upside of this was that the aboleth got hurt badly as well, so it wasn't interested in staying to fight. The PCs then rebounded from that room and re-encountered the vrock, which they had earlier decided was an illusion because, although it could clearly see them, it didn't emerge and attack. It killed Samaritha instantly and hurt the other two.
If I ran _Echo_ again, I would revise this. It doesn't make much sense, it's quite lethal, and it forces the players into "dungeon mode" when there's no need to do so. I really regretted having "proved" that Novelniss must be 14th level, because if the PCs believe that--and the fireball is strong evidence--they are going to go for help rather than enter the Echo.