October 2559. With the galaxy in the suffocating grip of a renegade artificial intelligence, another perilous threat has quietly emerged in the shadows: the Keepers of the One Freedom, a fanatical and merciless Covenant splinter group, has made its way beyond the borders of the galaxy to an ancient Forerunner installation known as the Ark. Led by an infamous Brute named Castor, the Keepers intend to achieve what the Covenant, in all its might, failed to: activate Halo and take the last steps on the path of the Great Journey into transcendence.