Tyrant
A tyrant was the absolute ruler of a political entity or specific area who ruled without constraints and oppressive, or in other words, committed tyranny towards his subjects.
In Klingon mythology, Molor was sometimes described as a tyrant. (TNG: "Rightful Heir", "Firstborn")
The 19th century song "A British Tar" featured the line "…or the tang of a tyrant tongue!" (Star Trek: Insurrection)
In the novel Commodore Hornblower, Napoléon Bonaparte was referred to as the "Corsican tyrant" by the parson of the village of Smallbridge. (DS9: "The Visitor")
During the Eugenics Wars on Earth in the 1990s, a series of Augment tyrants ruled over several areas. Khan Noonien Singh was described as "the best of the tyrants" and was the last one to be overthrown. (TOS: "Space Seed")
On Sarpeidon, Zor Kahn had the epithet "the Tyrant". (TOS: "All Our Yesterdays")
In 2370, Kubus Oak claimed he had never liked the "arrogant tyrant" Gul Dukat, even though the two had worked closely together during the occupation of Bajor. (DS9: "The Collaborator")
In 2372, Julian Bashir described Kira Nerys as a "vile little tyrant". (DS9: "Shattered Mirror")
In 2384, the Asencia from an alternate timeline established herself as the tyrannical ruler of Solum, opposed by the Uprising led by Ilthuran with the help of Asencia's own younger self. This ultimately led to the Vau N'Akat Civil War as the people rose up against her. With the help of the USS Voyager-A and the USS Protostar, Asencia was overthrown and imprisoned for her crimes. Wesley Crusher suggested that Ilthuran would then become the planet's new leader and lead his people into a better future. (PRO: "Ascension, Part I", "Ascension, Part II", "Brink", "Touch of Grey", "Ouroboros, Part I", "Ouroboros, Part II")