With the situation for season of the splicer coming up all the time let's explore the possibility to say Savathun is playing everyone.
What if Osiris isn't corrupted/hidden away/anything other than normal?
Let your theories fly. Mine is simple:
Savathun is a master of trickery. Perhaps she intended for us to focus on one guardian who responded to her song like Osiris and has instead been doing something behind our backs the entire time within the Vex network. Perhaps Mithrax was wrong about Quria being the center of the vex curse and she intends to plunge the last city into the vex network and reign chaos on people.
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<I think he's corrupted a little. A bit of influence thanks to the proximity of her machinations to the City's denizens. However... I never thought he was possessed. I think the writing around him is too obvious, as if Bungie WANTS us to figure out what should be a real twist. That's not Goddess of Deception writing, that's "learnt your first trick" writing.>