So I finally decided to actually watch Fate/Zero after years of backlog, and I remembered a fate fic I started reading years ago but never finished. I think it was on fanfiction.net but not entirely sure and couldn't find it again.
From what I remember:
Shirou somehow travels back in time to Arturia's time. Except he's early and manages to draw the sword from the stone shortly before Arturia would have. Merlin shows up and takes him aside and accuses him of being a timetraveler. He's also pretty angry/annoyed that his plans for Arturia were derailed. (Merlin may or may not turn into a dragon to scare/threaten Shirou... Not entirely sure that happened in this fic)
And that's about it. Not sure what it was called or how long it was. It's been at least 1-2 years since I read it, but it's probably older given my crappy memory.
Any help finding it would be appreciated!