We've known since September that the first piece of Destiny 2 DLC would be called Curse of Osiris and that it would take players to Mercury. Today, as part of its lead-in to Paris Games Week, Sony showed off the trailer for the expansion, and it looks... well, pretty dang cool if I say so myself.
Story-wise, Curse of Osiris will focus on you finding the titular Guardian and helping fend off an invading force of Vex from both the past and future. It'll introduce Mercury and its Infinite Forest region, provide players with new story missions and Adventures, as well as the usual run of new weapons and armor. There will also be new multiplayer maps for you to assert your dominance on, and new co-op activities for when you're feeling more charitable.
As for Osiris himself, he may seem like an all-new addition to the series, but he's actually been hiding in the wings for some time. There was of course the Trials of Osiris mode in the original Destiny, which players could only participate in if they proved themselves to be the best of the best in the Crucible.
Through various entries in the Grimoire (you remember the Grimoire, right? That out-of-game encyclopedia you had to read if you wanted any story from vanilla Destiny?), we know that Osiris was once a member of the Vanguard, like Zavala, Cayde-6, and Ikora. He was well-respected, serving as the Speaker's apprentice, and built a dedicated group of followers known as the Cult of Osiris.
He served as Ikora's mentor, and inspired other Guardians to seek out legends such as the Vault of Glass. Unfortunately, his obsession with dark powers led the Speaker to exile him, and he has not been seen since, though he has found ways to let his existence be known.
Will we see a darker bent of the Light? Will this fabled Guardian be friend or foe? We'll find out when Curse of Osiris arrives on December 5.