Is it a WoW expansion? A Diablo 3 expansion? A dessert topping?
On August 9, Blizzard filed a trademark for the name "The Dark Below." As is often the case with trademarks the listing is vague, simply describing it as a "computer game software" with "virtual currency and items."
It's anyone's guess as to what "The Dark Below" is, but popular opinion is declaring it as the Diablo 3 expansion that Blizzard promised last year. The developer will also reveal a cinematic at Gamescom that will "capture the attention of the Heavens, Burning Hells and all the shadowed places that lie between."
However, WoW fans have pointed out that a Cataclysm boss, Ozumat, is dubbed "Fiend of the Dark Below." Additionally, a non-canonical Warcraft RPG book describes The Dark Below as "a place inhabited by demons, devils and infernal creatures, from where some sorcerers take their power directly and sometimes have a patron beast to foster their magic."
It seems likely that we'll get a proper answer at Gamescom on August 21, but until then, fans will just have to speculate.
Source: MMO Champion