Place of Origin:
Engines of War
Interstitials was the name given by Cinder to a small population of mutated Time Lords living in the Death Zone during the Last Great Time War.
They were the outcome of the experiments of Rassilon, President of the Time Lords, whose aim was the construction of a possibility engine, a machine to question about decisions to take during the conflict. In order to this, he retro-evolved the timeline of a Time Lord, connecting it to the Time Vortex and forcing it in a loop of iterative regeneration. Rassilon left dozens of these experiments wandering in the Death Zone, until he managed to build his engine with Borusa.
Three of them helped the War Doctor and his companion Cinder to free Borusa from the Tower of Rassilon. (Engines of War)
APPEARANCE AND ABILITIES
The Interstitials had no constant appearance, since they shifted between their different incarnations. Due to their connection to the vortex, they were able to see the past and the future. Thanks to this, for instance, they were able to avoid rendezvous with the wild beasts of the Death Zone. (Engines of War)