The Grand Serpent

His Supremacy the Grand Serpent was a political figure on Inston-Vee Vinder's home planet. True to his name, he had the ability to project a serpent-like creature out of his body before reabsorbing it, being part of a rare binary demi-species; a species composed of two living forms. He interfered with UNIT using the name Prentis.

The Grand Serpent confirmed to the Thirteenth Doctor that "the Grand Serpent" was not his birth name, but rather a title he had earned, many years prior to their meeting. (TV: The Vanquishers)

Biography
The Grand Serpent met the Alforia to discuss terms to shelter them, in return for them providing "food and safety" for generations. After his new guardian, Inston-Vee Vinder spoke out of line, he dressed him down and ordered him to be silent from thereonin.

Once the official meeting was concluded, he ordered Vinder to cease recording and then demanded the Alforia hand over five dissidents being sheltered in their territory to face trial, and for another four, all relatives of his critic Frey Sampor, to suffer a fatal accident. Vinder reported the incident, resulting in him being reassigned to Observation Outpost Rose. (TV: Once, Upon Time)

Having been removed from power at an unspecified point in time, (TV: The Vanquishers) he positioned himself on Earth in 1958 in order to assist the founding of a new organisation known as UNIT. In 1967, after being identified as an alien by General Farquhar, he began a streak of assassinating those who stood in his way. In 2017, he encountered Kate Stewart, informing her that UNIT funding was being cut. When she retaliated by threatening to expose his history with the organisation, he attempted to kill her by leaving a bomb in her house. Later, he ordered UNIT to prepare missile strikes against various Earth cities before informing the Sontarans, with whom he had struck a deal, that everything was ready for their invasion of Earth. (TV: Survivors of the Flux)

He interrogated one aspect of the split Thirteenth Doctor upon her capture however as foiled by the arrival of a second aspect, who contained him. The Serpent escaped and reported the escape to the Sontarans, who reprimanded him. Whilst the Sontarans departed Earth for their final gambit at the second Flux event, he pursued his vendetta against Kate, tracking her down to the Williamson Tunnels. There he was cornered by Vinder and Kate, who forced him through one of the doors and destroyed it afterwards, stranding him on an asteroid with nothing surrounding it. (TV: The Vanquishers)

Behind the scenes
Some writers have noted numerous similarities between the Grand Serpent and the Mara, introduced in TV: Kinda.