Instructions
Play any part of "Halo: Reach" except the theater. Any game, even a custom one, will earn "cR" (credit) toward your character's ranking and ability to purchase items.
Achieve the rank of "Inheritor." This is the highest rank in the game and requires over 20 million cR to unlock. The Haunted Helmet does not become available until this rank is reached.
Purchase every other helmet in the game. This means every base helmet and every variant available.
Purchase the Haunted Helmet. Once the helmet is made available at the Inheritor rank, and the prerequisite of purchasing every other helmet is fulfilled, the helmet can be purchased. Fortunately, after all that work, the helmet costs only 1 cR.