There's a story where Luna doesn't wake up and, in order to help, Twilight tunes her magical frequency to Luna's, forever linking them. Anyone know the title? Much appreciated.

Dreadnought

The only such story that comes to mind is Oldnew Luna.

That seems less of a romance and more of a dark story. I kind of like it.

6083406
I agree. I read that story. its the one hes looking for

6083411

I think it gets more romance-y in the sequel. It's been a while since I've read the whole thing, though.

There is also this story.

You could be looking for Oldnew Luna or Friendship and Frequencies though the second had yet to be completed