Solitract

The Solitract was a consciousness or a form of energy. It came from before the universe and stopped the creation of the universe from being possible by preventing the various elements from coming together. The universe was able to exile the Solitract to the Solitract plane. There the consciousness would be alone, forced away from the universe so it would never learn how the universe had come about.

The Solitract was able to create a portal to the universe in Norway, 2018, and baited people to stay there in order to learn about the universe because it wanted to be a part of it. To do this, it created an image of Trine and Grace O'Brien. As its universe started collapsing from having too many incompatible life forms, the Solitract sent everyone but the Thirteenth Doctor back through the portal when they rejected it.

With only the Doctor left, the Solitract took the form of its favourite image, a frog. While in this form, it spoke to the Doctor with Grace's voice. However, the Doctor recognized that her presence was still causing the Solitract plane to destabilize which would kill both her and the Solitract unless she returned to her universe. Recognizing that the Solitract was just lonely and not evil, the Doctor admitted that she wished she could spend more time getting to know it, calling the Solitract her friend and "the maddest, most beautiful thing I've ever experienced and I haven't even scratched the surface."

The Doctor asked it to release her so they could both live on and the Solitract could "keep on being brilliant by yourself." The Solitract told the Doctor that it missed the universe and everything in it and the Doctor promised that they could be friends forever if the Solitract released her. The Solitract agreed and promised that it would dream of the Doctor "out there without me." As the Doctor left, she blew the Solitract a goodbye kiss.

After making it back to her world, the Doctor told the others that (while she doesn't know if the Solitract survived) it won't be returning to the universe. (TV: It Takes You Away)