The Seriphia Galaxy
Near Gallifrey’s galaxy
The Apocalypse Element
Dalek Empire (audio series)[which?]
The Seriphia Galaxy, also designated Galaxy 17A53, was a galaxy neighbouring the one that contained Gallifrey. It was four times the size of the Milky Way and home to over six hundred billion stars.
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 and kept from engulfing the universe through the use of the Eye of Harmony.
The Daleks accelerated time to ensure that the galaxy reformed, allowing the Daleks to establish a power base within it, with control over a million Skaros. (The Apocalypse Element)
The Daleks launched two invasions of Mutter’s Spiral from this galaxy 2,500 years apart. The Great Catastrophe didn’t destroy all Dalek technology in the Seriphia Galaxy. (Dalek Empire I[which?], Dalek Empire II: Dalek War[which?], Dalek Empire III[which?], Dalek Empire IV: The Fearless[which?])
The Assault on Seriphia was an event at the start of the Last Great Time War. (Hostiles)