"Dungeon Synth is a subgenre of dark ambient music that emerged in the late 1980s and early to mid 1990s. The genre employs aesthetics and themes typically associated with black metal and applies it to dreamier ambient songs. The style is often compared to video game music, as it is occasionally applied in those settings."
And while what wikipedia is says is true, there are many contemporary artists on bandcamp as well. These sounds range from medieval minstrel party music to fantasy cornputer dungeon crawler scores. For the most part, this genre seems to lend itself to background music throughout a tabletop RPG campaign.