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The possibility engine was a cybernetic machine built by Rassilon on Gallifrey during the Last Great Time War, constructed with Borusa at its heart. It could be consulted about decisions to make in the conflict, since it was able to see the entirety of all-time. It needed a Time Lord whose timeline was retro-evolved. One of the effects of this was that Borusa was in an iterative regeneration cycle, changing between each of their past and future incarnations. (PROSE: Engines of War)
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Experiments for its construction led to the creation of the discarded race of the Interstitials. The eventual functioning specimen worked thanks to Borusa and was hidden in the Tower of Rassilon, in the Death Zone.
The War Doctor, Cinder and three Interstitials brought the possibility engine on the Doctor's TARDIS and, eventually, to the Tantalus Eye; there it was able to bring into existence a timeline where the Daleks had never come to the Tantalus Eye. (PROSE: Engines of War)