Spoilers. This story takes place during Season 5's ending episode, "The Cutie Re-mark".
Starlight can't be defeated. But then again, she can't win either. Both she and Twilight and trapped in an eternal time loop. After realizing this, Starlight alters her spell to transport Twilight and Spike beyond time itself. Unfortunately she's dragged in as well and taken to the place where time ends. In that endless nothingness where even time is meaningless and realities merge she and Twilight find...
Celestia, Discord, and Chrysalis playing cards and betting on the lives of mortals. And it's not as funny as it sounds.