This story is a sequel to Midsummer's Eve
Twilight has been dating Princess Luna for some time. She feels that it might be time to take the next step in their relationship. But even if the proposal goes according to (elaborate, checklist-strewn) plan, forces Twilight knows nothing of may disrupt their happy future profoundly.
The final conclusion of the Mythic TwiLuna trilogy, following Spring Tide and Midsummer's Eve.