The Will of the Prophets
From Halopedia, the Halo wiki
The toy only appears in Remastered graphics mode. The achievement is worth 5 Gamerscore and can be unlocked on any difficulty.
Near the beginning of the level the player will pass through two pistons into a large room where several Grunts and Jackals are being attacked by Sentinels. Move through this room across a small bridge and into a second, similar room. As the player enters the room, Tartarus will tell the player, "You're getting close to one of the shield generators. Many of my Brutes have fallen attempting to take it down. Let's see if you fare better." On the left side of the room there are a series of conduits sticking out of the wall above the player's head. Use a grenade jump to get up to the ledge (using the Sputnik skull will make this easier). The toy is in the far conduit on the left side of the room.