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 mean Savathun is basically Aizen from Bleach controlling things in the background. Literally based on what people in the Lore are saying about Osiris he has been acting differently for a while now. EVEN SAINT-14 THINKS SO. He doesn't need to be Savathun but he Literally looked into a Cryptolith so if the current Osiris is still the real Osiris he is definitely being manipulated by her. Doing things behind the Vanguards back, bringing the Crown of Sorrow to the last city, wanting to capture and detain Quria in the last city... Dude is either INSANE or he is being controlled. He also according to the leak ~Spoilers~ helped FWC open a vex portal in the city... Osiris wouldn't do that because HE KNOWS TO FEAR THE VEX. He has been studying them for decades. So I throughly believe that Osiris is being manipulated by a team of Xivu Arath (Cryptoliths poisoned his mind just like the others in Season of The Hunt) And Savathun is pulling the strings watching us. They even directly say Ikora gave Osiris access to everything we have on her. So this is the perfect time for her to destroy or manipulate info we have on her. She is trying to destroy the only order the Last City has in order to use the Traveller to free her of her Wyrm and live forever.