The Edge

The Edge of Existence was a symbolic, metaphysical place in the Doctor's mind where each incarnation went before regeneration. It was represented to the Thirteenth Doctor by a barren cliff with a telephone pole. Here the Thirteenth Doctor interacted with the Guardians of the Edge, manifestations of her previous identities who maintained their own individual personalities while acting together as one being. They told her that she was determined to not have passed it yet, and that if she did, there would be no return. (TV: The Power of the Doctor)