Eternity is something that can never be reached. Even Equestria's princesses won't live forever. The end, however, comes much sooner than anypony expected.
Twilight went into Fluttershy's cottage to pick up her Nightmare Night costume. But she left with a deeper appreciation of life, death, and her pegasus friend, thanks to a Gila monster in a broom closet.
After Rarity's death, Doctor Whooves Takes Discord, Celestia, and Fleur Dis Lee several billion years into the future, to watch the long-princessless sun engulf the planet Earth. Somehow, that makes things better.
One-Shot. In a world where ponies live enslaved, where 'cutie marks' are both a symbol of rebellion and of hope, one immortal hero, 'Mare-Do-Well', arises to lead ponykind to freedom. This is the end of one, the beginning of many.
Sometimes, when we're really sad, and all alone, all we need is a friend, somepony to listen to us and care. Especially if what we're telling them is something important and close to our hearts.