Falkus was an artificial moon of Skaro. Falkus orbited Skaro three times every twenty-four Earth hours. It was the second moon of Skaro and its name was a Kaled word that meant "new dawn," (AUDIO: Daleks Among Us) but according to another account itis the first moon of Skaro. (AUDIO: Restoration of the Daleks)
History
During Davros' youth, both it and Omega Mysterium could be seen clearly from his father Nasgard's house on Drammankin Lake. Elsewhere on Skaro, they were obscured by the high levels of radiation in the atmosphere. (AUDIO: Innocence)
The Doctor once battled the Klade on Falkus, accompanied by "a tall man and a dark skinned woman." The Eighth Doctor did not remember the event, either implying it had been erased with his memories or had yet to happen. (PROSE: Father Time)
During the Last Great Time War, Davros was imprisoned on Falkus following his resurrection by the Dalek Time Strategist. (AUDIO: Restoration of the Daleks)
Behind the scenes
Origins
Falkus was created by the Daleks based upon Davros' plans. The city under its surface was a living machine. It could repair and service itself, hear, see and feel. The Daleks also built Falkus with defensive capabilities should it ever be attacked.
The Daleks who built Falkus had their memories wiped so they did not know its true purpose. (REF: Doctor Who and the Daleks Omnibus)
Purpose
Falkus served as a sanctuary for the Dalek race and all Daleks would think of Falkus with a sacred quality although none of them knew of its true function. Only the Dalek Supreme knew Falkus' purpose: a planet-sized Dalek gene-bank capable of creating thousands of Daleks in mere seconds.
Falkus was created as the Daleks' last hope in the face of extinction. The living mind of Davros resided within the heart of Falkus, ready to be placed in a new body and start the production in the factory should the Daleks ever become extinct. The Dalek Supreme would visit Falkus every hundred years in a one-man shuttle to make sure Falkus was still running. (REF: Doctor Who and the Daleks Omnibus)