Osiris. It's a name that players of Destiny and Destiny 2 should at the very least recognize. The "Trials of Osiris" multiplayer event from the first game is named after him. He's been referenced repeatedly in both games, but characters rarely speak of him openly.
Who is Osiris? What's his curse? And why is he the focus of the first expansion pack for Destiny 2, Curse of Osiris? Let's break it down.
According to Bungie's extensive Destiny lore, Osiris was a human Warlock and one-time apprentice to the Speaker. His heroism in one of the earliest battles fought after the construction of the City allowed him to rise to the position of Vanguard Commander — the same office now held by Zavala. Highly respected and fiercely skilled, he was also the mentor of Ikora Rey, current representative of the Warlock class among the Vanguard leadership.
Things went sideways for Osiris when he became obsessed with solving the mysteries surrounding the nature of the Traveler, the Guardians, and the Traveler's enigmatic enemy, the Darkness. He was particularly interested in prophecies surrounding the arrival of Oryx — which came true, in The Taken King — and the secrets of the Vex.
Over time, he came up with theories that other Guardians considered blasphemous, and he began using City resources to dig deeper into his obsessions. Those obsessions drove him mad, to the point that the Speaker was forced to depose him from his Vanguard position and exile him from Earth. It appears that he still has his Light powers, though.
Officially, the Vanguard has no idea what happened to him after that, beyond a vague report that he went to Mercury. But based on the just-released trailer for Curse of Osiris, it seems that at least one Guardian has secretly been keeping tabs on him: his protégé, Ikora Rey. Given his interest in the Vex, it shouldn't be surprising that Osiris turned up on the closest planet to our sun — a planet now used by the Vex as part of its universal hive mind network.
In Destiny 2, players learned that Mercury was being mined to the point of breaking apart by the Cabal, to fuel Ghaul's sun-killer weapon, the Almighty. It's unknown how much of it was broken off from the planet proper, but the Curse of Osiris trailer suggests that it's still at least partially intact.
So what's Osiris' curse? That's anybody's guess, but the trailer makes it seem as though it has something to do with an impending invasion by Vex. It's been rumored for a while that the Vex have access to time travel, possibly even seeing through time to adjust their tactics; the trailer is the first confirmation that this may be true. Perhaps in coming to understand the Vex, Osiris began to see through time as well, and that has something to do with his curse?
There's one more wrinkle to this story, though. Some time later, another Guardian from Osiris' time, an EXO named Saint-14, ventured to Mercury to find Osiris and learn what he was doing there. But Saint-14 vanished during this mission and no one knows what happened to him. It's possible that he could still be on Mercury, searching for or working with Osiris.
What happened to Osiris, and what he knows about the Vex and possibly the Darkness as well, will be revealed when Curse of Osiris arrives on December 5th.