"special announcement" for the conference.
According to the invite, "You are invited to join Blizzard Entertainment at a press conference on the first day of gamescom 2013. We're making a special announcement that's sure to capture the attention of the Heavens, Burning Hells and all the shadowed places that lie between."
It sounds like something related to Diablo, though what the "special announcement" about a Diablo game could be is puzzling. A Diablo IV announcement is unlikely, and the game has already been announced for next-gen consoles.
The conference will be held on the 21st at 11:30am CEST (10:30am GMT, 5:30am EDT, 2:30am PDT), Stay tuned.