Once Bungie has brought Destiny 2's servers back online, Curse of Osiris will be ready to go. "We want Destiny to be a world you want to be a part of". The studio has not yet released full patch notes for update 1.1.0; we'll amend this article with details once they're available. The Curse of Osiris DLC features a brand new storyline, involving Osiris himself (who was previously only featured in the original Destiny off screen), on top of a new Strikes, a new hub zone on Mercury, and brand new Raid content. He has been spoken of in the game before, but Curse of Osiris marks his first in-person appearance to date. New Crucible arenas and co-op activities are also being added.
The expansion is now live on PS4, Xbox One and PC, so no matter which version of Destiny 2 you own, there's plenty of new content to check out (providing you already preordered the Expansion Pass for $34.99).
The official description reads: "The Vex are on the march, and our only hope to stop them may lie with Osiris, the notorious heretic and most powerful Warlock in the history of the Last City".
Bungie are also increasing the level cap as well as new Raid experiences called Raid Lairs - these are supposed to be more intense and hard raids that exist "in and around" current Raids in the game.
It seems that the expansion might be just the excuse you need to return to Destiny 2 when it releases on the 12th of December 2017.