Osiris' story in Destiny 2
Osiris has been one of the most polarising characters in Destiny. From the early days of Destiny 1, Osiris has been an important character in the Destiny storyline up until Witch Queen.
At one point in time, Osiris was the commander of the Vanguard and was Ikora Rey's mentor. However, he was exiled from the Last City due to his obsession with the Vex, much like Toland's obsession with the Hive.
Here's a quick recap of the story of Warlock Osiris all the way up to Witch Queen.
The story of the Warden of the Infinite Forest in Destiny
Osiris trained under the legendary Iron Lords Felwinter and Nirwen, only to surpass them in no time. Although Osiris never joined the Iron Lords, he commanded a similar level of respect regardless.
As the days went by, Osiris kept neglecting his duties as the Vanguard commander due to his obsession with the Vex. His obsession, combined with the neglect of his duties, led to his exile from the city. Before leaving the Last City, Osiris left his student Ikora Rey with the difficult task of rebuilding the Warlock Vanguard.
Osiris also held council with the Queen of the Reef and The Nine over many important matters during this period. Due to his cult-like personality, he ended up gathering a lot of followers. These followers were known as the Disciples of Osiris, and they organized a tournament in his honor, known as the Trials of Osiris.
The legendary Warlock was influential in defeating Pantopes, Infinite Mind, and several other Vex entities during his time in the Infinite Forest. During his prolonged stay in the Infinite Forest, Osiris tried many different approaches to save his friend Saint-14, who had succumbed to the Vex inside the Infinite forest.
After the fall of the Infinite Mind, several timeways opened up in the Infinite Forest. The Cabal, recently having lost in the Red War in Destiny 2, tried to gain control of these timeways in an attempt to reverse their losses. The Guardian and Osiris get together to stop the Cabal from controlling these timeways.
Not only did they defeat the Cabal, but they also ended up saving Saint-14 by altering the timeline in such a way that he never died in the first place.
Osiris' return and Sagira's death in Destiny 2
Osiris got in touch with Zavala during the Arrival in Destiny 2 and sent him a Fang of Xivu Arath and then went to the Tangled Shore to find out more about Caitl trying to restore the Red Legion.
However, he found an organic tower that had a device inside it. The device lured Osiris, seducing him with thoughts of violence. This device is believed to have been designed by Xivu Arath, the Hive goddess of War in Destiny. Sagira managed to stop him from investigating the device.
On his return to Earth, Osiris discovered similar signals from the Moon and went on an expedition to investigate them. On the Moon, he was able to reverse some Hive communication networks where he learned about Savathun's decision to go into hiding and Xivu Arath's new conquest in the Solar System.
During his investigations on the Moon, he accidentally came across an emissary of Xivu Arath, the High Celebrant, who was conducting a ritual with Voshyr and Kinox. Osiris interrupted the ritual, killing Voshyr in the process. However, The High Celebrant was much more powerful than Osiris. He wanted to turn the Warlock into a Wrathborn, but Sagira sacrificed herself.
Although her sacrifice saved her Guardian, it left him ghostless. Savathun chose this moment to possess him. Disguised as him, the Witch Queen entered the Last City and infiltrated the Vanguard, gathering more information about the Light.
Following the events of the Endless Night in Destiny 2, Savathun revealed herself to the Vanguard. She promised to return Osiris to them if the Vanguard exorcised her Worm Familiar. The exorcism was successful, but Savathun escaped once again, swapping places with Osiris.
As for the ongoing story in Season of the Haunted, the Warden of the Infinite Forest lies in slumber, recovering from his injuries.