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 residing 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 when she discovered the incubation level. The Dalek Supreme manipulated her into sharing power and resources between the Dalek City and the newly-built Thal City by providing her access to Dalek science, creating the ionising towers that could reduce the radiation present in Skaro's atmosphere. In reality however, the Supreme sought to renew hostiles and wipe out the Thals, covertly activating the now gestated embryo and placing them into casings, ordering his forces to carve a tunnel to the Thal City.
Fifty cycles after the Thal-Dalek battle, the First Doctor and his companions returned to Skaro. Though initially commiting to its ruse of peace, the Supreme dropped the act when the Doctor, Tryana and Ian Chesterton discovered the secret tunnel. When Ian managed to flee back to the Thal City, the Supreme activated its new army before it donned its armor and exterminated Tryana. Taking the Doctor prisoner, the Supreme advanced to the Thal City, meeting with a Dalek scout who had taken Susan Foreman hostage as a means to blackmail the Doctor into revealing the secrets of the TARDIS. As the group continued onwards, the Supreme was fought off by Ian Chesterton and Tryana's son, Jyden, using a discarded Dalek casing.
Ordering the Doctor recaptured, the Supreme led the assault on the Thal City, seeking out Damadus, the High Leader of the Thals, before the Doctor took control of the ionising towers and turned them against the Daleks. Retreating back to the Dalek City, the Supreme addressed his troops before Ian overloaded the power feed flowing from the Thal City into the Dalek City. Its systems overloading, the Dalek City exploded with the Supreme proclaiming that the Daleks would prevail before it perished. (AUDIO: Return to Skaro)
Like the many Dalek Supremes that would succeed it, the Supreme had more individuality than most Daleks, having a dominating personality that demanded total obedience. He seemed to have something resembling a, rather sadistic, sense of humour, taunting Tryana about his deception of her and later punctuating her extermination by agreeing that she would not have to watch her people die. He was also adept at manipulation, managing to trick Tryana for two years that he wanted peace as he secretly rebuilt and expanded his influence.
Like all Daleks, the Supreme had an inflated sense of pride, believing that the Daleks, despite not having left their planet yet, were the rightful masters of the universe. The Supreme seemed to derive pleasure from personally exterminating people, to the point of boasting about his kills to the Doctor and leading his troops from the front lines. However, the Supreme was not so prideful as to keep fighting a losing battle, ordering a retreat when the ionising towers were turned against his forces.
In a sharp contrast to later Supremes however, this one seemed somewhat forgiving of failures, accepting his attendant's explanation for why the videoscopes hadn't detected a later party of intruders without complaint. (AUDIO: Return to Skaro)
Behind the scenes
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.