Most of these are not going to be either under-the-radar or recent, but neither of those were stated criteria, so hopefully no one will mind. :)
Not sure where you draw the line on what constitutes "metal". I don't think most metalheads (at least, those that listen to modern metal) would count much of what I listen to as anything harder than "hard rock", but....
Anyway, I'll try to tread carefully in that respect. :p

- A.A. Bondy: "How Will You Meet Your End"
- Most stuff I've heard by ††† (a.k.a. Crosses) would fit. My favorite song of theirs with which I'm familiar is "Trophy" (though the version I like best is a non-album version that's not on YouTube).
- Chris Cornell: "Steel Rain" (Extremely depressing and a little spooky!)
- Bush: "In a Lonely Place" (Joy Division/New Order cover)
- Joni Mitchell: "Woodstock" (Also worthwhile is Chelsea Wolfe's cover, which is still haunting, if slightly less enthrallingly "weird" than Mitchell's original)
- Cruel Black Dove: "Wasting" (I much prefer The Vandelles Remix, if you can find it somewhere) & "Offer"
- Black Prairie: "Red Rocking Chair"
- Fever Ray: "If I Had a Heart" (I get the impression that anyone with a penchant for darker-hued electronic music is way ahead of me on this recommendation, but better safe than sorry)
- A fair bit of Eric Heberling's excellent score for The Elder Scrolls: Daggerfall qualifies. Here's a playlist that I found of "night music" (using remade tracks, though they seem fairly close to the originals); dunno how much of the dungeon music is in there, but most of that would be right on target for this thread

I'll post more some other time, if I remember.