It had been years since Twilight Sparkle took over ruling Equestria and Celestia and Luna retired, but the two ex-princesses still continue to visit and hang around Canterlot. It was then Flurry Heart approached the oldest pony alive- Celestia- about Tirek, and Celestia began to tell the same tale she told Twilight all those moons ago. Tirek and his brother Scorpan came from distant lands to steal Equestrian Magic, however Scorpan came to appreciate Equestria and the creatures within- befriending a young Starswirl the Bearded- and pleaded for him to cancel the mission. However, Tirek refused under the belief that Scorpan had betrayed their kind, and in order to save Equestria Scorpan warned the princesses of his intentions. Though Scorpan returned to his homeland, Tirek had been locked in Tartarus for a millenia before escaping and facing against Twilight Sparkle and her friends, getting defeated and sent back to Tartarus before Discord summoned him along with Cozy Glow and Chrysalis, all three getting defeated and being turned into stone and so on.
… Okay, MAYBE discovering Queen Chrysalis and Cozy Glow were reformed may be a bit. Hard for him.