This is the being that noticed Phoebe sloppily opening the way to visit our dreams, and letting others visit our dreams. Iskra was able to tell that it is from the astral plane.
After talking to Dr. Brightmine and there are some creatures in the astral plane that interact with dreams and can cause nightmares. This type of creature feeds on the misery and misfortune of its targets, but cannot kill anyone from the astral sea. If the being managed to find its way to the material plane, however, that’s another matter.