As the focus of the second episode of Series 8, Into the Dalek, Rusty was a one-of-a-kind, a character that initially seemed like it could be the universe’s only example of a Dalek that was morally good. That is, until the Twelfth Doctor discovered that it was simply suffering a malfunction, cured the problem, and instead accidentally taught Rusty to hate the Daleks. At the end of Into the Dalek, we see Rusty leave for parts unknown, but what happened to this unique Dalek?
Escaping the Dalek Fleet
At the end of Into the Dalek, Rusty leaves the Human hospital ship Aristotle and joins the rest of his kind in their saucer, and he then presumably stays undercover until he reaches a strategic position. Interestingly, early drafts for Into the Dalek depict Rusty self-destructing to destroy the saucer in a manner similar to the Metaltron from Dalek, though this was cut from the final episode.
What we do know is that Rusty would survive and live among for the Daleks for a while before defecting and waging an unending war against his own race. Rusty would go on to destroy countless Daleks, and he became somewhat of a legend both among his own people and in the wider universe in general. According to the Twelfth Doctor, Rusty would go on to live for billions of years, slaughtering Daleks and becoming a quasi-mythical figure.
Waging War against the Daleks
After meeting the First Doctor in Twice Upon a Time, the Twelfth Doctor would eventually reunite with Rusty after travelling to his fortress on Villengard. Rusty has clearly been busy since his last encounter with the Doctor, as there are dead Daleks strewn around the entrance to his tower, and he has been living there for so long that the scattered Dalek survivors have adapted to live outside their casings and latch onto the faces of humanoids in order to feed, implying that Rusty had been there for hundreds of thousands of years, if not millions.
A former weapons production facility, Villengard was perfect for Rusty’s purposes, and he not only installed himself atop a tower with external weapons, but he also tapped into huge Dalek databases in a bid to become one of the most intelligent life forms in the universe. The Doctor would tap into this knowledge to learn more information about the Testimony, eventually discovering its benevolent nature. The Twelfth Doctor was then teleported away and Rusty is left alone once again.
Twice Upon a Time is the last time we see Rusty, so what he gets up to after this and what eventually becomes of this Dalek is unknown. His unending hatred of the Daleks that has lasted for billions of years will undoubtedly motivate Rusty to continue his crusade against his own kind, and perhaps he will appear in a story in the future and shed some light on how he has managed to evade destruction and wreak havoc on the Daleks for so long.