Basim is an Assassin that teaches Eivor (the playable character and Viking) how to be an Assassin. The Maguffin of the game is “Valhalla” but it’s actually a stasis device that the ancient people used to try to avoid their apocalypse. It didn’t work as they thought it would but remained dormant. Eivor and Basim fight near the machine and it activates and traps Basim there for centuries. In the modern timeline, the player character Layla finds the corpse of Basim at the machine but accidentally activates it, the machine releases Basim’s corpse and captures Layla. Layla happened to be holding the staff of Tresmisgustus (the maguffin from AC Odyssey that grants eternal life) so when Basim’s corpse touched it, he was reanimated. So now Basim appears to be the main character for Mirage in both the past and present timeline. There is even more with Basim’s story from Valhalla that I left out, some big details actually, but that’s the gist of it