Moral Event Horizon: In-universe, Celestia believes she crossed this when she chained The Sun and forced Her to bend to her will in order to avoid setting at an extremely inconvenient time, then kept Her bound for over a thousand years.
Slow-Paced Beginning: The first couple chapters plod and meander through weddings and arguments, and it takes a while to even get to the main plot. But when the story gets there, you suddenly realize why this fanfic is so well liked.
The entirety of the final chapter. It will crush your soul to dust, then turn that dust into beautiful sculptures.
Make that the whole of the "Eternal" two-parter before the epilogue, then the Epilogue itself. It's a really long, but oh so beautfiul series of Tear Jerkers. Or, hell, make that the entire thing when taken as a whole. The whole fic is just...beautiful, especiallly when you've taken the time to see just how much Twilight, Celestia and Luna had to go through, to reach the conclusion at the end.
