Regent Worf was the undisputed and merciless leader of the Klingon-Cardassian Alliance in the primary Mirror Universe in the Star Trek spin-off series, Star Trek: Deep Space Nine where the Terran Empire had fallen.
Much of his early life is unknown, but it's assumed he rose to power in the Alliance by committing acts of genocide and possibly killing his superiors to rise to the top of the Alliance until he became the Regent, which is the title given to anyone who rules the Alliance.
Frustrated by the constant failure of Kira Nerys, who not only lost the station, Terok Nor to the Terran Rebels, but was captured herself, he resolved to recapturing the station himself with a task force of Klingon and Cardassian Ships. They failed against the Rebels when they fought the Regents fleet with a new rebel warship, the ISS Defiant, this universe's counterpart to the USS Defiant in the Prime Universe.
Worf was forced to retreat. Worf would capture Kira Nerys later on with the intent on executing her for her failures. But she seduced him into letting him live when the Ferengi Grand Negus named Zek from the Prime Universe went to the Mirror Universe for business opportunities in that universe. But he was captured by the Regent and was told that in exchange for his freedom, that he would have to provide them with a cloaking device from the Prime Universe.
Worf would get the cloaking device and have it installed in his ship by the Prime Universe's Ferengi Engineer, Rom, who is the son of Zek's girlfriend, but Rom sabotaged the device and it caused the ship to have a total systems failure in battle.
The ISS Defiant took advantaged of this and crippled the Regent's ship. Worf had no choice but to surrender. After he surrendered, he would throw a tantrum on his bridge stating that he should have killed the Ferengi the moment they stepped aboard as Kira slipped away. He and his crew was later on captured by the Rebels. In the novels that followed the series Worf was held as a prisoner of war by the Alliance.
The former regent was well treated during his captivity. The Klingon-Cardassian Alliance collapsed in the coming years due not only to humanity rising up against the Alliance but also from internal pressures. A new Galactic Commonwealth made up of humans and many of the species that made up the Federation of the primary universe - rose to become a power in the region.
After a treaty was signed between the Commonwealth and the Klingon Empire Smiley O'Brien came to Worf's cell with a D'k tahg. Offering the weapon to Worf the grateful former regent proceeded to end his life.