The trademark could be for a new game or unannounced IP, but it's more likely the title of an upcoming expansion to one of Blizzard's big existing titles. Smart money is on Diablo III, since Blizz did tease its August 21st Gamescom press conference by saying it would, "capture the attention of the Heavens, Burning Hells, and all the shadowed places that lie between." We also know that Blizzard plans to show off a brand new cinematic at Gamescom -- cinematics tend to come with either new games or new expansions.
Any guesses on what The Dark Below might be?
[Thanks to Sharvis for the tip!]
[Update: Our sister site WoW Insider argues convincingly that this particular registration was merely a hoax.]