Geas

A Geas is a Forerunner term, meaning a "genetic command" imposed on an organism or species of organisms. Through the use of a geas, an organism can be given a set of subconscious orders that will be passed on to their children, lasting several generations in some cases. Highly skilled Forerunner Lifeworkers, such as the Librarian, are capable of creating a geas.

Forerunner scholars hypothesized that the Precursors may have imposed a geas on the Forerunners themselves.