Dark Lord of Skaro was a title held by Davros, the creator of the Dalek race. Skaro was his homeworld (TV: The Magician's Apprentice) and the base-of-operations for the Daleks. (TV: The Witch's Familiar) Colony Sarff referred to Davros by the title when speaking with Ohila. (TV: The Magician's Apprentice)
Other references
During the Last Great Time War, Dalek Caan referred to Davros as "Lord Davros". (PROSE: The Third Wise Man)