Cryptum

The unidentified sentinel was seen in the Halo 4 E3 sneak-peek trailer. It seems to have been buried underground until the UNSC Infinity flew overhead. It is one of the many various new Forerunner sentinels seen on Requiem.

The sentinel appears to be the main antagonist of the story-line, as Cortana and SPARTAN-117, John, refer to it as "him", suggesting a previous encounter. It also appears to be in conflict with the Master Chief and Cortana, suggested by their race to the crash site of the UNSC Infinity.