Lord Tirek is a male centaur, the older brother of Scorpan, and the main antagonist of the season four finale. He is a villain who escapes from Tartarus, steals magic, and seeks to rule over Equestria.
Tirek is defeated and banished back to Tartarus by the Mane Six through Rainbow Power magic acquired from the Chest of Harmony. He is later released from his imprisonment by Grogar to join his alliance, along with Queen Chrysalis and Cozy Glow, to defeat Twilight and her friends and conquer Equestria.
(and, given what happened to him during Sombra's last defeat, I have it that he's in a rather weakened state. You know, preventing him from thinking he can just repeat what he did last time and expect a different outcome.)
(Well how I go about roleplaying Sombra is, after his latest defeat, he finds a way to pull himself back together and returns to the living world. Perhaps Tirek senses a familiar dark presense and goes to investigate it only to find Sombra has returned?)