Hmm, it sounds vaguely like Silmaril by lintamande and LuminousAlicorn... but that's not really a fic, nor is it on ao3, so I don't think my guess is right?
What I'm thinking of is really short, like, a one-shot with a couple pages to it, and I've heard Silmaril is a longform WIP, so that's another mark against it. LuminousAlicorn and lintamande doing a collab is one hell of a recommendation, though.
...yeah, that's definitely not it. (I can recommend Silmaril! --just, block out a fair bit of time for it. It's big. )
