The main storyline will center around the legendary Warlock Osiris and his ties to Ikora Rey, one of the main characters in the series.
You'll explore Mercury's "Infinite Forest" in new missions, earning new themed weapons, armour and gear, while also engaging in new co-op activities (Strikes, presumably) and enjoying new competitive multiplayer arenas, "and more", according to the Xbox.com listing, as shown in the screengrab below.
The new Mercury destination will come with new Adventures, and new story missions.
On Wednesday, September 13, Bungie will open the doors to its first raid in Destiny 2, which is called "The Leviathan". The listing doesn't provide much of details apart from a short insight into what Destiny 2 players will be doing and the kind of content to expect.
Destiny 2 is now available on PS4 and Xbox One, with a PC release scheduled for October 24, 2017 on PC.
Kotaku revealed before Destiny 2's release that the expansion would have that title and would include a new Patrol Zone on Mercury and a new social space, making it more substantial than the first game's initial expansion. Mercury's mapped area, part of Destiny 2's new feature set, is named the Infinite Forest. "It's real!" and while knowing that we're going to Mercury is an incredible feeling, we still don't have a release date.
Update: Bungie has confirmed the existence of Curse of Osiris.
A representative for Activision told Polygon that information about Curse of Osiris will be posted to Destiny's official website later today.