I'm expecting we'll see something sent by
Jubilex, long thought to be the Abyssal Lord of Oozes, but in fact he was merely an Aspect of
Ghaunadaur, the Drow God of Abominations.
Well, only if we're using the D&D lore for Jubilex rather than the Pathfinder background. In this case, I hope it's the D&D side, however, as the slime's presence would tie in very well if it originated with the Aspect of a Drow deity, considering they appeared to have been the enemies of the Serpent Folk. Also, D&D 3.5 Jubilex is a CR 19 threat, which would make a fitting boss encounter for the party.