The Daleks were involved in a long and costly war with the Movellans. They realised that neither the Daleks or the Movellans had any advantage as they were both machines/robots. A group of Daleks returned to Skaro in search of Davros their creator as they hoped he’d help them find a way to defeat the Movellans.
These Daleks only appeared in the 1979 TV story Destiny of the Daleks.
Pictures above are from the TV story titled Destiny of the Daleks