The song is Exile, by Enya. The AMV I'm looking for isn't on Youtube (though two other Sailor Moon/Exile amvs are).
It must have been around 1996-2003, ancient in Internet years, and I fear it has been deleted from the web forever.
I was certain I downloaded it from here, but alas no.
It started off with Serenity dying and ending with Sailor Moon crying at the mirror to the dark world, where Mamoru was taken.
It's more for nostalgia's sake, it probably doesn't hold up to AMVs these days, but I recall it was well timed.
Likewise, for another one which took me a while to find, only to see it has been deleted:http://www.animemusicvideos.org/members ... php?v=1064
If anyone can help, I'd appreciate it greatly.