“The Decree of Terminus is one of the greatest unsolved mysteries of the Warhammer 40000 universe. At least, it used to be.
It is a secret last order given by the Emperor of Humanity to the Gray Knights, the most mysterious order of Space Marines, before the Emperor was placed on the Golden Throne after being mortally wounded by Horus at the end of Horus’ Heresy. Only the Supreme Grand Master of the Gray Knights can open the Decree of Terminus and find out what exactly it provides. In fact, it is a “Pandora’s Box” Warhammer 40000 games and fans have been trying to guess its meaning for years.
But the mystery came to an end when Games Workshop released the new Codex Gray Knights — a book containing a lot of backstory, images for use in the board game, and new rules. Among them is a passage that reveals the “Decree of Terminus”. It reads The ultimate duty of the Grey Knights is to either kill or re-seat the Emperor on the Golden Throne if he ever tries to leave it, be reborn, or ascend to the divine state.
A passage of the text tells us that the Decree of Terminus does not provide any instructions on how the Gray Knights should do this, if they ever have to. They would probably have to overcome the resistance of the Adeptus Custodes, the Emperor’s super-powered bodyguards, who are stronger than the Space Marines. How tells IGN based on Reddit posts, fans are unhappy with the fact that the secret has been revealed. But this is not the first and not the last time that the creators of a fantasy universe have made What the fans like.