Alicorns are not immortal - merely long-lived.

On her deathbed, Celestia confesses to Luna about the loneliness she suffered during their thousand years apart - and of Celestia's secret plan to make up for her greatest failure as Luna's sister.

This is an expansion of my entry into Flashfic 150 - September 2019 with the prompt 'Bittersweet'. I felt the idea had legs beyond 150 words and took it a bit further.