He's "of The Dark Below". The Dark Below is a place. You can try and argue it means Burning Legion are coming from under the ground, or that Azjol Nerub is deep beneath Northrend or w/e but I'll just laugh when the expansion is out. It's not confirmed to be an expansion no but people said the same thing about MoP when it was trademarked, at around this time of year as well I should add. People will be saying "IT'S NOT CONFIRMED YET" until the day of Blizzcon. Then we shall see.