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 find it very unlikely that Savathûn used Osiris without having influence over him, that’s assuming she’s not wearing an Osiris suit like a skin puppet. Before Season of the Splicer maybe you’d have a case, but this Season Osiris has made a clear attempt to play every side in the political turmoil of the Last City and gone behind the backs of both his partner Saint-14 and his student Ikora. Honestly the fact Ikora didn’t go to Eris, likely because of her involvement with Stasis, is going to end up being a critical error.