The Dalek Empire detonated the Apocalypse Element within the galaxy, causing widespread destruction at eighteen light-years Gallifrey-relative and would be destroyed over six hours. However, the unpredictable nature of the element sped up the destruction, but the fireball was contained in an accelerated time continuum through the combined use of the Time Lords’ Eye of Harmony and Dalek thought crystals, keeping it from engulfing the universe.
The Daleks had planed this acceleration to ensure that the galaxy was reborn after the destruction, allowing the Daleks to establish a power base over the newborn worlds - over a million Skaros. (AUDIO: The Apocalypse Element)
The Daleks subsequently launched an invasion of the Milky Way from their powerbase in Seriphia. The Dalek Supreme spoke to Susan Mendes from Seriphia and boasted of how the Daleks had created it using the Apocalypse Element. (AUDIO: Invasion of the Daleks) Kalendorf’s Great Catastrophe finally put an end to their occupation in the Milky Way, destroying all Dalek technology except for an outpost in the Seriphia Galaxy which diverted the self-destruct order into the mind of a single drone, which also absorbed the minds of the Dalek Emperor and Susan so was declared the new Supreme. (AUDIO: The Demons) The Daleks rebuilt in Seriphia and approximately two thousand years after the Catastrophe plotted their return to the Milky Way. (AUDIO: Dalek War: Chapter Four)