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.

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 noted of 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)

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 detail anything about the event. It was first alluded to by Captain Jack in The Parting of the Ways but not named.

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, 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."