The entities Carl senses menacing behind him at the dream's edge — two obvious, a third well concealed.
As Carl tries to recede from Katia's mind and feels the event horizon of the dream looming, he registers something behind him: "another presence menaced. Two presences. Three presences, but one well hidden." He turns to face them instead of following Katia, since she has to do her dream alone.
Only one of the presences is accounted for — Shi Maria, the spider bound to Carl's Sternum Tattoo and the intended target of the dive. The identity of the others, and especially the concealed third, is unknown, and it feeds Carl's earlier suspicion that the convenient availability of Mordecai's potion means the Showrunners or The AI have a hand in what is about to happen. The two sides close on Carl and Katia like a pair of hands clapping together in a clumsy attempt to catch a bug.
Formless menacing presences perceived at the boundary of the dream, one of them deliberately concealed.
The unknown company waiting inside the memory dive, one more than Carl bargained for.