Basically looking for traditional heavy metal with really elaborate guitar work and a dark atmosphere, but preferably with stronger vocals [at least compared to Terry Jones of PA].

...anyone? There's gotta be SOMETHING like this out there!

I'm going to rec a band that you've already heard............ Satan (UK).... *sips wine*

Yeah, that joke is old, but Satan was the first band to come to mind.

If you like Mercyful Fate a lot a like their style check out Portrait from Sweden. They are very similar in many aspect and have a particularly riffy style.

I think I heard a Portrait song once. I'd honestly rather just stick to MF, and I don't know if that was the effect they were going for. The style just doesn't seem to be one that works well when imitated, it's just too unique.

Try Morgana Lefay's Maleficium album and Root's The Black Seal if you haven't.

Going to shamelessly copy/paste my response to a similar query in the heavy/power/speed help thread:

Have you heard any of the French 80s heavy metal scene? Contemporary, but similar. The big three are:
Sortilege
http://www.youtube.com/watch?v=qTuXPoeOphs

Blaspheme
http://www.youtube.com/watch?v=5IIutTgq3aA

High Power
http://www.youtube.com/watch?v=1WzU9CJnLRU

Remember the last one is from 1983!

Well, you've got Judas Priest....

Maybe Memento Mori, if you don't mind it a wee bit slower.
You could try Force of Evil. Has the same guitarists. Can't speak for it as I haven't heard any of their stuff though,