One of the best and by some accounts only melodies in the entire genre of black metal is this short guitar riff from Burzum that has inspired many covers.
The original is an intermission track, a sweet thing that breaks up the harsh tone of the rest of the album. Different groups have extrapolated this brief moment into full songs. This one by Nicrotek is my favorite.
But there are tons of them. This one is closer to the original with better guitar work.
This one is very slowed down, played on a paino in a more dungeon synth/ambient style.
Here's one that's clean on an acoustic guitar
There's an old 90's interview somewhere that Varg says that the orc is sad in his hatred of everything. I've always imagined it differently though.
Oak-gods wither, oaths warp, wasted; Short sounds shimmer, shadow-shrouded; Cross stands crowned where cairns crumbled.