Neverwhen, also known as the Neverwhen flux or flux device, was a Time Lord weapon that created a place outside of normal time and space. Any beings that were in its sphere of influence experienced time phasing, where their biology and technology continually changed along their evolution due to a time flux.
The potentiality warp was the Neverwhen's "engine room", its control matrix which existed both inside and outside the Neverwhen, beyond most planes of existence.
During the Last Great Time War, the weapon was deployed in orbit of a strategic planet being fought over between the Time Lords and the Daleks, trapping a thousand battalions within the new reality. Having spent at least "weeks" within the Neverwhen, both the Time Lords and Daleks were contaminated: were they to leave, instability would run throughout the universe.
The Dalek Scientists on Asteroid Theta knew about the Neverwhen, where Ollistra obtained a data stamp copy of the coordinates and programmed them into The Doctor's TARDIS (AUDIO: A Thing of Guile) as it was used to travelling to unusual places. She planned to use it to break the time lock on Skaro and, along with the Anima device, cause civil war at every point in Skaro's history. However, the War Doctor changed the parameters of the potentiality warp, so that when the weapon was orignally deployed, it destroyed the Time Lords, Daleks, and itself. (AUDIO: The Neverwhen)