From Halopedia, the Halo wiki
The suits are tailored to Spartan snipers, fitted with emissions cloaking, heavy-duty waste recycling, and redundant rebreathers for extended combat excursions in any environment. While designed for extreme-range operations the suit still is functional in both mid and close range. Deadeye can be optimized for extreme long-range target purging for the extreme end of sniper-oriented Spartan armor. With the smart-linked VISR software with movement-distance calibration, Deadeye has been updated to assist snipers at ranges that greatly exceed traditional firing accuracy.
The helmet has an enclosed PZ-238 rebreather system. The VISR has a point-isolated, VISR focal software used for extreme-range targeting. All Deadeye helmets also have a small piece of alloy from ships resting in the wreckage above Tribute's poles.
The Deadeye variant was manufactured by Hannibal Weapon Systems. It was originally tested in Tribute's ship graveyard in Tribute's northern pole, and was designed for use by experienced Spartan marksmen. During the testing the Spartans were placed in zero-gravity to affect long-range combat against exoatmospheric drones hidden among the wreckage.
Although Deadeye is rare in the field, the armor is highly regarded by its advocates.
Unlock: The helmet was a pre-order bonus through Best Buy or Play.com. 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
Halo 5: Guardians skin
List of appearances