Update: Bungie's marketing director Eric Osborne confirmed Curse of Osiris as the first expansion, but doesn't clarify if all the reported details like the release month.
Curse of Osiris is not a leak. It's real! :)— Eric Osborne (@UrkMcGurk) September 8, 2017
Original article: According to a leaked Microsoft store listing, we now have details regarding Destiny 2's first post-launch expansion, the Curse of Osiris. The expansion will include a new planet, loot, missions, and story that focuses on the titular warlock.
Curse of Osiris will open up the planet Mercury for the first time as part of the game's story content. The new Mercury zone is called the Infinite Forest, and will presumably come with its own set of Adventures, Hidden Sectors, Patrols, and Vanguard. The story will revolve around the "legendary Warlock" Osiris who you presumably have to rescue from the Vex based on current in-game story tidbits in Destiny 2. Osiris is Ikora Rey's teacher, and she also plays a role in the Curse of Osiris expansion.
"Destiny 2 Expansion I: Curse of Osiris continues your Guardian's journey with all-new story missions and adventures in a new destination, Mercury. Journey through time and space to learn the secrets of Osiris, avert a dark future, and rebuilt the tites between the legendary Warlock and his greatest student—Ikora Rey."
- Explore Mercury and its mysterious "Infinite Forest."
- New story missions and adventures
- New themed weapons, armor, and gear to earn
- New cooperative activities
- New competitive multiplayer arenas
Previously, Mercury and Osiris were only encountered through the Trials of Osiris multiplayer Crucible event. If players ran a flawless 9-0 run, they would gain access to the Lighthouse on Mercury. However, a report from Kotaku last week says that the Lighthouse will be open to all players as a new social space in addition to the Farm on Earth. Meanwhile, the Trials of Osiris will be replaced by the Trials of the Nine which begins on September 15.
As part of the same Kotaku report, Curse of Osiris will apparently launch in December, while the official listing has the expansion set for "Winter 2017". The first Destiny 1 expansion, The Dark Below, also debuted three months after the game's release, though it didn't add a new explorable planet like the Curse of Osiris.
You can purchase the Expansion Pass for Destiny 2, which includes the first and second expansion (the latter set for Spring 2018) for $34.99.