Promising "all-new story missions and adventures" on Mercury, Destiny 2 Expansion I: Curse of Osiris will apparently see you travelling through "time and space" as you seek to learn the secrets of the eponymous Warlock Osiris and avert a dark future from befalling the galaxy. The main storyline will center around the legendary Warlock Osiris and his ties to Ikora Rey, one of the main characters in the series.
Osborne wrote on Twitter "Curse of Osiris is not a leak". The new Mercury zone is called the Infinite Forest, and will presumably come with its own set of Adventures, Hidden Sectors, Patrols, and Vanguard. (Bungie announced yesterday that Trials of the Nine, the equivalent event in Destiny 2, will begin September 15.) Osiris is a "legendary Warlock" in Destiny lore who served as a mentor for the Warlock Vanguard, Ikora Rey, according to the product listing for Curse of Osiris.
What awaits Destiny 2 Guardians on Mercury remains a mystery, though there are probably a few hints given in the sequel's campaign. There is also going to be a lot of co-op and multiplayer content.
In addition to the story quests, the DLC also includes new cooperative missions and maps for the player-versus-player Crucible mode. The Flashpoint activity will move over to Nessus, Xur will make his first appearance with a collection of Exotics for purchase, and a new experience called Trials of the Nine will begin on Friday.
We will update you as soon as Bungie reveal what is included in the Curse of Osiris Expansion, and when it is due to release, as soon as we know. The game somewhat sputtered out of the gate with a lackluster campaign and didn't really hit a solid stride until the release of the third DLC, The Taken King.