Mantle

The Mantle is mentioned in the Terminals found on the Ark and the second Installation 04. While no exact definition of it exists, it is heavily implied that it is the source of their "authority", and is the reason why they fight the Flood - to protect their "charges" from the Parasite. It has been implied, albeit in a derogative manner, that the Mantle was passed down to them by another species, speculated to be the even more mysterious Precursors, and handed down to Humanity.

Trivia

 * The Librarian shows that she is particularly doubtful about following the Mantle as shown in one of the Terminals on the Ark
 * This could be similar to the Mandate of Heaven and they both use the prefix "Man-".
 * It could also refer to the planet like object in the legendary ending of Halo 3.