The Terminus Decree is a set of ancient instructions written upon millennia-old parchment and sealed within a mysterious artefact of the Grey Knights Space Marine Chapter. This artefact goes unrecorded in all the libraries of the Imperium, for it has been kept secret from all but the Supreme Grand Masters of that Chapter. It is said that the Terminus Decree contains the final secret of the Chapter, a secret so great that it could destroy the foundations of the Imperium -- or provide salvation in its darkest hour.
Deep within the Chambers of Purity, within the Grey Knights fortress-monastery on Titan, locked away in the chamber said to hold the tomb of Malcador the Sigillite himself, rests a simple wooden box, embellished with a golden seal. Within this box is the ancient parchment that contains the instructions known only as the Terminus Decree. Only a Supreme Grand Master of the Grey Knights knows how to open the box, and he will do so only when all hope for the future of humanity seems lost. The Terminus Decree is the ultimate sanction of the Grey Knights, a secret so vast it could bring the Imperium to its knees, or save it in its darkest hour. The exact nature of the document is unknown, and the only clue to its contents lies in the box's golden seal. It is whispered that it is the exact match of another seal, found only in one place in all the Imperium of Man's many scattered worlds: within the Imperial Palace, upon the Emperor's Golden Throne.
- Codex: Grey Knights (7th Edition), pg. 21
- Codex: Grey Knights (5th Edition), pg. 9