The Terminus Decree is an ancient scripture or instruction written on a simple piece of parchment and locked away within the Chamber of Purity on Titan, making it a sacred relic to the Grey Knights Chapter.
Such is the power of the Terminus Decree that it is to be opened only in humanity's darkest hour, when all hope for the race is lost. What happens once the box is opened and the decree is spoken by the Supreme Grand Master is infinitely dangerous and will either save mankind from certain defeat or doom it to oblivion for all eternity. In this capacity it is the ultimate sanction of the Grey Knights, to be used only as a last resort, when all other options have failed.
The exact nature of the Terminus Decree is unknown, as its very existence in known to only a single soul in all the galaxy, yet rumors abound as to its ultimate purpose.
Some believe that it will summon forth some ancient and terrible force to battle and defeat the enemies of mankind. Others think it will purge the warp of the Chaos Gods and their minions, and bring about a new age for mankind, free of their daemonic influence.
Still others believe that once read aloud by the Supreme Grand Master, it will perform its rite and bring to a halt the ancient and arcane machinations of the Golden Throne, ceasing their life-giving support of the Emperor's long dead body and freeing his soul to be reborn anew. (This is the most "likely" theory as it is said that the golden seal which rests upon the box containing the decree is one of a matched pair, its partner found only in one place: on the Golden Throne itself).