Dalek invasion of the galaxy

Human historians in the post-Time War universe, following the Siege of Trenzalore, understood that a full-scale Dalek invasion of the galaxy of Mutter's Spiral was launched after the failure of the Exxilon initiative.

Background
The Exxilon Gambit was the final action of the Third Dalek War, (PROSE: The Secret Lives of Monsters) which followed the 26th century Second Dalek War. (COMIC: Pureblood) It took the form of numerous expeditions by opposing parties to the planet Exxilon to acquire the rare mineral of parrinium. (TV: Death to the Daleks) Human historians postdating the Siege of Trenzalore indicated that the Daleks' germ warfare campaign which culminated with the Exxilon Gambit came multiple centuries following the 26th century Dalek War. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) The Time Lords, however, identified the Exxilon Incident as taking place in the late 27th century. (PROSE: Dalek Combat Training Manual)

The invasion begins
Following the events on Exxilon, the Dalek Supreme Council decided that the time for discretion had ended. The galaxy was still reeling from the Daleks' space plague and any further delay would give them time to recover. The invasion was launched and only the continued preparedness of the Combined Galactic Resistance kept them from overrunning the whole of inhabited space in the first assault. The Daleks scoured the life from whole planets. Hospital ships treating plague victims were converted into troop carriers. Conscription was introduced on a thousand worlds. Bitter enemies buried ancient rivalries to unite against the Daleks. The once great empire of Draconia splintered and fell before the Dalek onslaught. Humanity endured, but every day more ground was given and more worlds fell to the Daleks. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

A Good Dalek
Rusty was a standard Dalek drone until one battle left him drifting alone in the depths of space; unknown to him, his power source had cracked and was leaking radiation that disrupted his shell's programming to keep him from feeling anything but xenophobic hatred towards non-Daleks. Seeing the birth of a star triggered a realisation in Rusty of the inevitability of life returning despite the efforts of the Daleks. From this, Rusty saw Daleks were evil and needed to be wiped from existence. Eventually, he was found by the Combined Galactic Resistance, and brought aboard their ship the Aristotle (TV: Into the Dalek) in the Ryzak solar system. (PROSE: The Secret Lives of Monsters) They attempted to open the shell, hurting Rusty. Badly hurt already, Rusty asked for help in recovering so he could exterminate his own race. Interested in the idea of a moral Dalek, the crew promised him medical attention.

The Doctor, Clara Oswald, Journey Blue, Ross and Gretchen Carlisle were shrunk and ventured into Rusty to try to fix the damage that was killing him. Using his sonic screwdriver, the Doctor repaired a crack in Rusty's power source that was leaking radiation and killing him. However, without the radiation affecting him, Rusty reverted to his original Dalek programming, restored to full power and went on a rampage, killing several soldiers and sending a distress call that attracted a nearby Dalek flying saucer. Upon urging from Clara, the Doctor decided to attempt to restore the changes once more. He had Clara reactivate the Dalek's suppressed memories so he could see the birth of the star again and be reminded of the universe's beauty.

Clara succeeded and the Doctor telepathically linked with Rusty to show him more from inside his own mind. At first it seemed the Doctor had succeeded, as Rusty again saw beauty and divinity. However, Rusty also saw the Doctor's hatred of the Daleks, which resonated even more strongly with him. Rusty saw the Daleks as an evil which must be, in true Dalek fashion, exterminated. Rusty then went on a rampage against his own kind, slaughtering the Daleks attacking the Aristotle and fooling the saucer into retreating by falsely telling the other Daleks that the Aristotle's self-destruct had been activated. Rusty left with the Daleks to continue his efforts against them, telling the Doctor that although Rusty himself was not a "good Dalek", the Doctor was a good Dalek. (TV: Into the Dalek)