The Sith Emperor is the primary antagonist of Star Wars: The Old Republic.
According to the tie-in novel The Old Republic: Revan the Sith Emperor was originally born on a planet called Nathema, under the name Tenenbrae, the illegitimate son of the Sith Lord Dramath. At age six, he began showing signs of force sensitivity. When this happened, the man who thought he was his father confronted his mother, and upon a confession attacked her. Tenenbrae then snapped his father's neck, and tortured his mother for betraying the family. He made the others in the village bow down to him.
Eventually, Dramath, hearing of Tenenbrae's activities, went to see if his son was strong enough to serve the Sith. Tenebrae then killed him, taking his position at thirteen years old, under the name Lord Vitiate. Eventually, he called many Sith Lords together to Nathema, announcing some powerful discovery he had made. Once they were there, he performed a ritual which instantly sapped every living thing on the planet of it's force, killing every lifeform, human, alien, insect, and plant alike, creating a wound in the force which would never heal.
As the now immortal and nigh-unstoppable Sith Emperor, he took control over a large portion of the galaxy, which he ruled for millennium. He led a war against the Republic, known as the Great Hyperspace War, but was defeated by the Jedi Order. The Sith retreated, regaining strength slowly over hundreds of years. During this time, two Jedi, Malak and Revan, stumbled across the Empire on Dromund Kaas. He broke their minds, sending them back as the first guard, to lead an offensive against the Republic. The events of Knights of the Old Republic happened then.
Eventually, as Revan and Malak did not return, the Emperor assumed they had been defeated by the Republic. It was not until Revan returned in search of his memories that the Emperor learned the truth. The Sith Emperor captured Revan, and has been keeping him prisoner ever since, in a stasis field, fully conscious, but ageless.
The Emperor has not been seen as of late, with the Dark Council operating on their own, leading some (such as a cult known as the Revanites) to assume that Revan defeated the Emperor, and took his throne.