Twilight finds herself dreaming of the events from the "Even Mountains Erode Over Time" timeline but doesn't realize that they are actual memories . Luna comforts Twilight and argues with Celestia on telling Twilight what her (and Sunset's) ultimate destiny is and whether the choice they had to make was fair.
Exact show setting is around the time of the "Castle Sweet Castle" episode where Twilight is feeling alone in her new castle.