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 like the theory that Osiris has ‘Doctor-stranged’ all this and knew what he and everyone else has to do in order to beat Savathun. Whether that was being abducted by her/taken over or just acting weird is yet to be revealed but I do like the idea that he foresaw all of it.