Terminus Decree

The Terminus Decree is an ancient parchment located in the Chamber of Purity in the fortress of the Grey Knights on Titan with the instruction that is the Terminus Decree. This artefact goes unrecorded in all the libraries of the Imperium, and is a secret to all but the Supreme Grand Masters of the Grey Knights.

It is said to hold a secret so powerfull that it could bring the Imperium of Man on its knees or save it on its darkest hour. None of the contents of this document is known save that its golden seal matches the one located in the Golden Throne of the Emperor. Thus, it serves as an ultimate sanction to the Grey Knights only to be used as a last resort.

Source

 * Codex: Grey Knights (5th Edition), pp. 9