Canine Space

This is the bottom of the imaginary number space; the bottom of consciousness made from the dregs of corrupted information. Not even Moon Cell’s surveillance can reach this East of Eden.
The ground is covered entirely in velvet, but other than that it is an empty bed of nothingness.
This place lacks the concept of “an entrance” or “an exit,” as well as “limits” there is no means of escape.
Those who fall here are connected together in the flow of time, wandering aimlessly for all eternity, drifting in a sleep-like state.
BB adds a further special rule for those who reside here: “You must be on all fours.” She does this to force the protagonist to surrender to her will by abusing both his body and mind.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA