Lord Tirek

Tirek is a male centaur, the brother of Scorpan, and the main antagonist in Twilight's Kingdom - Part 1 and Part 2. He escapes from Tartarus in order to gain power by stealing magic. Prior to his appearance in G4, he was notable as the villain in Rescue at Midnight Castle from G1.

Personality
Tirek is generally depicted as evil, sadistic and narcissistic. He is shown to be a skilled manipulator, even more so than Discord. He is often seen to associate compassion with weakness as a result of his brother's betrayal.

Depiction in fan labor
Tirek began appearing in fanfiction long before the announcement of his appearance in Friendship is Magic. Interestingly, some of these works involve Tirek returning to power after coming back from being banished in Tartarus.

Occasionally, Tirek is depicted reuniting with his brother, Scorpan, following the events of the season 4 finale.

Other works, however, instead use Tirek as a plot device more than as a character. In these cases, Tirek has been defeated but lingering evil magic, in some way, leads to the conflict of the story.