Dominator invasion of Gallifrey

The Dominators launched an ultimately unsuccessful invasion of Gallifrey by "flood[ing]" the Capitol with Quarks. 39 of these Quarks were destroyed by Leela and K9 Mark I, (AUDIO: Time in Office) both of whom had became residents of Gallifrey following the Vardan/Sontaran invasion. (TV: The Invasion of Time) One of the Quarks was fitted with a bomb which heavily damaged the Capitol, necessitating the construction of a replacement building. Speaking following Borusa's Game of Rassilon, the Fifth Doctor claimed that there was never "any serious risk of us being conquered." (AUDIO: Time in Office)

Previously, the Second Doctor cited the Quarks, "deadly robot servants of the cruel Dominators", as an example of the evils he had fought to the Time Lords during his trial, noting that their attempt enslave a peace loving race. (TV: The War Games)