Her name is Lady Selenite

...selenite is a stone too, isn't it? ^_^

Some AU story behind this... Queen Beryl managed to find Usagi before Luna, but didn't kill unawakened princess and instead brainwashed her, turning Usagi into a fifth general - Selenite (probably as a revenge to deceased Moon queen). Later Beryl finds Mamoru and turns him into Dark Endymion, but guess what - true love can't be washed off so easily ^_^ (and when Beryl find that out, she will be very, very angry)))
Meantime Luna with other four senshi is searching for their princess, not knowing that she is one of their enemies now... Well, all would be fine in the end

UPD. A kind deviant Legal-Assassin-006 brought me link on a similar story - The Dark Side of The Moon sadly, it's unfinished for a long time, but it's a good story and you can still have a nice time reading ^_^

(Though converting Serenity there reminded me of Black Lady, and felt little OOC for me, but I probably couldn't manage better... I myself was thinking about honest brainwashing and the same happy-go-lucky clutz Usagi, just on a wrong side, and oh what a pain in the butt she was to seriously working generals... can't write at all, meh ^_^; )