The singer don't have to sound like King Diamond (in fact, it would feel a lot less "rip-off" if he didn't), I'm looking for bands which have the same kind of melodies and guitar-playing. Already heard In Solitude and Ram, but they're awesome.

Portrait from Sweden.

Forgot to mention that I've heard them too.

Try Fates Warnings' The Spectre Within if you haven't heard it yet.

Demontage's new album is very Mercyful Fate at times, chunky, evil riffs and barbaric savageness wrapped up in a lot of wrathful melody.