Tirek

A centaur from a far away land. He came to Equestria over a thousand years ago to absorb all of its magic, but his brother Scorpan befriended Starwswirl the Bearded and helped to stop Tirek's conquest. Celestia and Luna helped to trap him in Tartarus, but he eventually managed to escape in the modern day. Fortunately, Discord sent him right back to Tartarus, ignoring the centaur's attempt to turn him against his friends because it would hurt Fluttershy.
He's been mentioned several times as Tirek the Soulless, particularly by Spike, but he's never actually interacted with the main cast and his one appearance happened offscreen.
==Story==
===Thread 257===
[1] Mentioned by Celestia, among other creatures, as being locked away in Tartarus when asked by Alterjack who's in there: "A great demon named Tirek, who must never be released!"