From Halopedia, the Halo wiki
The helmet is designed to sync/dock with an SV module called the "LCX-20V1 SinoViet sight-augmentation module" for extreme-range reconnaissance. It does not mitigate operability, and can be slid back into a passive mode above the user's parietal plate. The helmet also has a low-set, concave rebreather apparatus.
It was manufactured by the Materials Group and tested at the Damascus Testing Facility. Over several years Materials was balancing out the needs of personnel for search-and-rescue operations, appropriately fine-tuning individual components like the long-range sensors and large toolsets like armor-fixed rescue implements. It was designed for the facilitation of search-and-rescue strike teams protecting ONI personnel, as well as critical government and military personnel from capture by Insurrectionist forces.
The armor is widely recognized as the best Mjolnir suit available for high-value target and hostage recovery operations.
Unlock: Unlocked by purchasing the War Games Map Pass. It is also available in the Halo 4 Game of the Year Edition.
Halo 5: Guardians
Unlock: Ultra Rare REQ card
In Halo 5: Guardians due to the "Body" having a predefined customisation this armor set permanently has the following parts from Halo 4:
Halo 4 skin
List of appearances