Mature content warning - There is a fair deal of violence in this story, as well as lesbian relationships. Also, there is a lemon chapter, but it's removed enough from the story where you can skip it without missing anything.
If, y'know, you're one of the two people on the internet that isn't interested in two girls doin' it.
http://www.shoujoai.com/fanfics/smoon/absence/
Absence is the story of Rei and by extention the rest of the Sailor Senshi. It's your typical SM story arc in theory - new baddies come, Senshi deal with them, baddies die, romantic pairing is happy together. In theory, asa pretty much all of those are tossed on their head. It starts with Rei, her feelings for Usagi, and her allowing her friends to think her dead as she goes to start a new life. From there, it spirals in a very dark rendition of the SM universe all the way to the gritty end.
It has many good points - it's immersive, long, well written and appealing to those whom like a good dark fic. The characters are reasonably on the mark considering the setting, and most of the new characters work well. The baddies in particular have a strong feeling of menace to them, which aides them loads throughout the story. This alone, even with any flaws mentioned below, makes it a worthwhile read. if a long one.
What bad features it does have are endurable, largely because of the above merits. There is a duo of new characters. They're vaguely annoying and hangonish, and probably should have been left behind far earlier in the story than they did. Secondly, I'll be honest - while the other Senshi aren't shortchanged, Rei is notably in godmode. She's not invincible, but she does lots of cool things, has the focus on her, kicks ass, etc al. It's a turn off if you're not a fan of Rei, I suppose. Finally, the most annoying and possibly damaging point is the ending. Without spoiling it, the author chickened out in favor of an ending against the grain of the fic she had set up.
Despite those flaws, it's still wonderfully enjoyable. If you're interested in Sailor Moon, shoujo-ai or darkfics, it's worth reading.
Heh, I can never forget the "Gadzooks! I've been stabbed! I'm dying!... oh, hold on, no I'm not" part. There's too much angsting, the arc where Rei went off on her own was... bad, but I don't regret reading it. I don't remember any demonization of Mamoru either, which seems to be de rigeur for Rei/Usagi canon-ish setups.