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 the Forerunners' "authority", and is the reason why they fight the Flood—to protect their "charges" from the Parasite. It has been implied, albeit in a derogatory manner, that the Mantle was passed down to them by another species, speculated to be the even more mysterious Precursors, and that it has now been handed down to Humanity.

Many Forerunners doubted the purpose of the Mantle, dismissing it as superstition and fairy tales. The Librarian, in particular, doubted that it was useful or benign, stating that she believed it responsible for the downfall of the Forerunners at the hands of the Flood. It is largely implied that this was indeed the case: the Mantle acted as a stumbling block to the progression of Forerunner society and ultimately was nothing more than an impediment.

Simply put, the Mantle was the Forerunner's belief that they had to preserve biological diversity in the galaxy, which would provide one reason for their heroic mass self-sacrifice by creating and firing the Halos in an effort to at least spare humanity.