Well... was hoping for more responses from others rather than me making some, but two come to mind that *may* apply, though likely not how you are thinking...
Both dark stories, well... *I* consider them such, even though I quite liked the first arc of the one, and the second one is one I still pull out and dust off to reread... Both center on Sailor Mercury.
In the first, Mercury and a Youma become lovers. She gets to watch as Sailor moon murders her...
SM 'Waking Ami' by PinaPoe - [Dld Date=01/01/2005] - [Dld Size=552kb] - [Dld File Cnt=17] - [Dld Status=Complete]
- - [MD Source=FF.NET] - [MD Rating=M] - [MD Word Cnt=108107]
- - [MD Summary= Throughout her life Ami has never had friends or happiness. Until Juni Asagai a troubled teacher with a dark secret and Makoto Kino a homeless teen looking to start over change everything for better or for worse. Shoujoai]
- - [Direct URL] https://www.fanfiction.net/s/1426855/1/
- - [Author1 URL] https://www.fanfiction.net/u/131272/
SM 'Love and Lament' by PinaPoe - [Dld Date=09/25/2006] - [Dld Size=689kb] - [Dld File Cnt=25] - [Dld Status=Complete]
- - [MD Source=FF.NET] - [MD Rating=M] - [MD Word Cnt=126311]
- - [MD Summary= In the midst of reclaiming memories of a past life and attacks from an insane Jadeite Ami Mizuno must find a way to reclaim her broken life. Her loyal friends valiantly stand by her side perhaps one will became something more. sequal to Waking Ami]
- - [Direct URL] https://www.fanfiction.net/s/2201806/1/
- - [Author1 URL] https://www.fanfiction.net/u/131272/
I never cared for the sequel as much, because I truly felt an awakened Ami that saw Usagi do that to her lover would *never* forgive her, or Pluto forgive Usagi for that matter.
In this other one... Mercury becomes an assassin, and still knows knowledge of the SM times. And though a series of events is forced to choose between her chosen profession and her supposed destined one.
SM 'Life's Blood' by Dro'gan NiteFlier - [Dld Date=08/28/2004] - [Dld Size=314kb] - [Dld File Cnt=9] - [Dld Status=Complete]
That second one especially I had always been sad was not continued further, I truly loved that story and plot.