Guardians (Halo)

Guardians are Forerunner AI constructs created to enforce Forerunner laws on troublesome worlds and keep them in line. In Halo 5: Guardians, Cortana awakens a number of them to assist her in her goal of claiming the Mantle of Responsability for the Created.

Pre-Halo
The Guardians were originally created by the Forerunners to oversee worlds they deemed troublesome. The Forerunners also attempted to use the Guardians against The Flood, but only proved to be ineffective against the parasite. The Forerunners later hid the Guardians on numerous planets throughout the galaxy.

Halo 5: Guardians
The Guardians were later awakened by Cortana to enforce the AIs' claim to the Mantle of Responsibility. Guardians soon activated on many human colonies, causing innumerable amounts of damage.

Halo 6
The Guardians will presumably return in Halo 6 as major antagonists.