An enormous, pink-and-teal water monster that eats minds. Originally from another plane.
There were at least two of them in the world, the one that killed Ixtalan and was later killed by Michka Imet, and its mate, which the Shepherd’s End kids encountered later and bargained with it (after beating it to within 11hp of its life, sigh) to return it to its home plane, as it called this world a trap. The second one stole most of Xalati’s memories of Ixtalan.
When Esmund opened a Door to let it go back home, they spied an enormous serpent, larger than a city, in the distance.
Later revelations hint that the “home plane” of the Deeplurker might well be Paradox.