After Tirek had been defeated by Twilight Sparkle and her friends, he was stripped of all the magic that he had stolen from everypony and sent back to Tartarus to be imprisoned.

Days turned to weeks. Weeks turned to months. Months turned to years. Years turned to centuries.

However, even after all that time, Tirek is still alive. He lies defeated and broken, yet he still refuses to give in.
His hatred for her will not allow himself to surrender to his fate and let that purple alicorn win. So he continues on, driven, his anger his only anchor against the darkness and solitude of his imprisonment.

It seems like Tirek is locked in more than one cage.