The Dalek Supreme resided in a large nutrient tank rather than a conventional casing, although the mutant could transfer itself between the tank and an alternative Black Dalek casing to become mobile. It was the "original leader" of the Daleks, who went on to survive (AUDIO: Return to Skaro) the Thals' invasion of the Dalek City (TV: The Daleks) by hiding in the secret incubation level; (AUDIO: Return to Skaro) According to another account given by Ian Chesterton, the original leader of the City, the Glass Dalek, was apparently killed when its casing was shattered in the original Thal assault. (PROSE: Doctor Who in an Exciting Adventure with the Daleks) Nonetheless, the Dalek Supreme reported being the one who gave all the orders, from Central Command, during the events of the Doctor's first visit.
Many cycles after the Thal-Dalek battle, the Dalek Supreme encountered the Thal's Chief Scientist, Tryana. The Dalek Supreme manipulated her into sharing power and resources between the Dalek City and the newly-built Thal City by promising to help her rebuilt Skaro. In reality, the Daleks sought to renew hostiles and wipe out the Thals.
When the attack on the Thal City began, the Dalek Supreme transferred itself from its nutrient tank into its black casing in order to lead the Dalek forces directly. It murdered Tryana and attempted to blackmail the First Doctor into revealing the secrets of the TARDIS. However, it was fought off by Ian Chesterton and Tryana's son, Jyden.
The Doctor, Ian and the Thals later overloaded the power feed flowing from the Thal City into the Dalek City, destroying the Dalek Supreme as well as all the other Daleks and the City itself. (AUDIO: Return to Skaro)
Behind the scenes Edit
Return to Skaro's account of this Dalek City leader as existing outside of a permanent casing, instead inhabiting a large glass tank, points toward this Dalek Supreme being the same individual as the Glass Dalek who ruled the City during the Doctor, Ian, Barbara and Susan's first trip to Skaro and whose casing was destroyed in the Thal attack. The existence of its unique black casing may also suggest a link between this Dalek and the Black Dalek in The Curse of the Daleks.