Obtained: Automatically after completing The Common Thread main goal
Ask Equitia about the location of each missing plaque
Speak with Equitia to get a hint as to who might be able to help you find each plaque.
Collect all four stolen plaques
Follow these guides to located each of the four plaques:
Return all four plaques to the obelisk
Head up to the Great Temple and place each plaque on the obelisk near the cliff overlooking the city. This will allow you to unlock the Great Temple and head inside.
Take Proserpina’s Crown
Make your way to the throne room then shoot Proserpina in the glass case to the right of the throne. Run over and pick up her crown. Now flee the temple and head back to her shrine where Sentius has created the portal. Use it to reset the loop.
Enter the Great Temple
Speak to Galerius to have him help the citizens of the city (make sure to have completed A Virtuous Man lead first) then return to the temple.
Persuade the God of the Underworld to abolish The Golden Run
Speak with the creator of the Golden Run and choose to toss Proserpina’s crown at his feet. Follow the dialog options to get him to stop The Golden Rule. This will lead to some more dialog sending you to the end of the game.