The Supreme Dalek was the Dalek who took the privilege of unleashing "maximum extermination" on Missy and Clara Oswald when they were captured and brought to the Daleks, and ordered for an energy weapon to destroy the Doctor's TARDIS. (TV: The Magician's Apprentice)
Later, the Supreme Dalek ordered the extermination of the Twelfth Doctor after he had stolen Davros' chair and mocked the Daleks, however, it failed. He noted how the Doctor was against using weapons only to be scalded by the Doctor, who wanted Clara back. The Supreme Dalek proudly professed that Clara was "not alive", which angered the Doctor greatly.
He later interrogated Missy, who had survived extermination by using her vortex manipulator at the last second, about Clara's whereabouts, who had also survived in the same manner. The Supreme Dalek then quickly silenced, as the regeneration energy harvested from the Twelfth Doctor by Davros flowed through Davros' life-support systems into it, along with all of the other Daleks, in an attempt to create Daleks with the powers of regeneration. The Supreme Dalek praised Davros for his achievement, only to begin panicking about the sudden appearance of decaying Daleks coming from the sewers, due to them also being restored.
When the Daleks from the sewers began to rise above to the city itself and destroy the Daleks as revenge, the Supreme Dalek demanded the Doctor to help them, but the Doctor and Clara escaped in the newly-reformed TARDIS, and the city was destroyed. (TV: The Witch's Familiar)
Befitting its title, the Supreme Dalek was the leader of the Daleks in the city of the restored Skaro. (TV: The Magician's Apprentice) Unlike a number of Daleks, including the Supreme Dalek identical to itself that commanded the Crucible, (TV: The Stolen Earth, Journey's End) he showed a strong degree of respect for Davros, and chose to help him by sending other Daleks to assist Davros when he cried for help, rather than shunning him; (TV: The Witch's Familiar) Davros himself noted, however, that despite their loyalty to him, the Daleks, including the Supreme, were independent of his control. He also interrogated and spoke on behalf of the Daleks as well as deciding the fate of their prisoners. (TV: The Magician's Apprentice) He was also keen on exterminating the Twelfth Doctor, quickly ordering his extermination when given the chance. The Supreme Dalek was also notably arrogant, initially dismissing the Doctor's demands in the belief that he would not succeed in defeating the Daleks. It was also willing to anger the Doctor further by proudly proclaiming that "Clara Oswald [was] not alive". Despite its arrogance and pride, the Supreme Dalek was not above demanding the Doctor's assistance when the Dalek City was being destroyed by the renewed sewer Daleks. (TV: The Witch's Familiar)
The Supreme Dalek's casing was identical in appearance to a number of Supreme Daleks from elsewhere in the Daleks' timeline, such as a Dalek Supreme from the Second Dalek War, (AUDIO: Out of Time) a Dalek Supreme from the Last Great Time War, (COMIC: Ambush) and the Supreme Dalek that led the New Dalek Empire onboard the Crucible. (TV: The Stolen Earth, Journey's End) Said casing resembled that of a bronze Dalek, but coloured red with gold sense globes, in addition to a golden grating section that featured four large golden struts at compass points. Its red dome was adorned by three large luminosity dischargers instead of the standard pair that was featured on most other Daleks. (TV: The Magician's Apprentice, The Witch's Familiar)