Bones Of The Reaper by steamfan
Gen Doom/Star Trek
https://www.fanfiction.net/s/6268033/27/Bones-of-the-Reaper

Excellent story about Bones actually being Reaper from Doom. The crew of the Enterprise find out when McCoy accidentally gets his skull cracked when he, Jim, and Spock are captured. When he awakens he is missing time and does not remember being McCoy. Instead Spock and Jim are introduced Reaper who helps them fight their way out. Later he explains who and what he was. Now though there is danger when idiots (cough, Komack, cough) try to reopen the sites on earth and Mars. The writer managed to integrate the Bones and Grimm characters without losing anything of our favorite snarky doctor. The surprising introduction of Bones' daughter and her comments is funny and sad at the same time. The characterization is done well with no one over acting and they even use some of his known quirks and tying them to the past. Jim of course can't wait to point out that Bones needed his big sister to help him. Discreetly of course since Bones already threatened him with painfully embarrassing physical.