Tenth Dalek Occupation

The Tenth Dalek Occupation was the final major event known to involve the Daleks before the outbreak of the Last Great Time War.

Disappearance of the Daleks
The author of A Brief History of Time Lords recorded that the Daleks vanished from time and space after the Occupation. Galactic folklore held that "they had gone off to fight a bigger war".

Subsequently, the Dalek Emperor launched the Dalek forces into the Time Vortex, beginning the Time War. (PROSE: A Brief History of Time Lords)

Jack Harkness, a 51st century Time Agent, noted that the Daleks were "the greatest threat in the universe" before their sudden disappearance, which the Ninth Doctor confirmed was so they could fight the Time War. (TV: The Parting of the Ways) Jack, who recognised the bronze Dalek saucers, originally believed that they had been destroyed (TV: Bad Wolf) and that the Time War was just a legend. (TV: The Parting of the Ways)

Return of the Daleks
In 200,100, the Dalek Emperor, having survived the Time War, launched an invasion of Earth with his army of Daleks. Rodrick, an Earth human, was skeptical as to the return of the Daleks, recalling that they had all disappeared thousands of years prior. (TV: The Parting of the Ways)

The first Daleks of the New Dalek Paradigm, who were created in 1941 by Dalek survivors, were intended to return to their "own time", which the Eleventh Doctor knew to be in "the future", where they were to "begin again". Ultimately, he failed to prevent the Daleks from escaping via a time corridor. (TV: Victory of the Daleks) The Daleks set their sights on Skaro, their home planet which had been left abandoned and in ruins following the Time War. Though an initial attempt at recovering the planet was averted by the Doctor, (GAME: City of the Daleks, TV: Asylum of the Daleks) the Daleks eventually succeeded in rebuilding and repopulating Skaro. (TV: The Magician's Apprentice/The Witch's Familiar)

Behind the scenes
The Tenth Dalek Occupation is mentioned in a few sources, but A Brief History of Time Lords is the first in-universe source, and therefore the first valid source, to both name and detail anything about the event.

Jack's line in The Parting of the Ways - "One minute [the Daleks are] the greatest threat in the universe, the next minute they vanished out of time and space" - could originally just have been a generic assessment about the overall danger posed by the Daleks. However, this line largely serves as the foundation on which the fictional history of the Occupation is built. A Brief History of Time Lords, plus other reference materials, later attributed this period to the end of the Occupation, thus making The Parting of the Ways the first story to allude to the Occupation, if retroactively.

In The Dalek Handbook, the Tenth Dalek Occupation is said to have occurred some time after the trial of the Master on Skaro, although the relevant dates are unknown. Collating details that can be gleamed from other stories, primarily The Parting of the Ways, as above, its brief account of the Occupation reads: "... the Emperor of the Daleks led his renewed race to become truly the greatest threat to the universe. The Daleks had grown in power and technological ability and now waged war on the cosmos, and especially on the human race, in a period of aggression that culminated in the Tenth Dalek Occupation. After which... nothing. The Daleks vanished from time and space."