Arcturus Mengsk a Terran from Korhal IV Leading the Sons of Korhal To Overthrown the Confederacy of Man. He Succeeded using Genocidal tactics abandoning Sarah Kerrigan, on Tarsonis, which resulted in her transformation into the Queen of Blades.
Disgusted by this, fellow rebel Jim Raynor cut ties with Mengsk and established his own group of rebels entitled Raynor's Raiders.
Upon the fall of The Confederacy, Mengsk established The Terran Dominion, where he crowned himself Emperor. However, he was not much different from the people he sought to replace and continued to rule via manipulation and terror. Raynor's Raiders constantly have tussles on with the Dominion on many worlds in their quest to overthrow Mengsk.
He had a son named Valerian Mengsk, who eventually led Dominion forces along with Raynor on a full-scale assault on planet Char in order to de-infest Kerrigan.