Chronal tumour

Chronal tumours were a symptom caused by the shells of Rassilonian Timeflies. The first Time Lord believed that they could bestow an "uncanny knack" for navigating the Time Vortex. The Eleventh Doctor believed that someone could use a chronal tumour to get into the Last Great Time War, at the risk of being neurologically pulverised. (COMIC: The Judas Goatee)

During the Last Great Time War, the Master operated on the brain of his TARDIS, causing a chronal tumour to protrude from the side of the console. (COMIC: The One) Alice Obiefune used the tumour to take the Master's TARDIS back into the Time War. (COMIC: Running to Stay Still)