Hidden weapons caches on Forerunner
From Halopedia, the Halo wiki
The hidden weapons caches on Forerunner are an Easter egg found on the Halo 4 mission Forerunner. Exclusive to Legendary difficulty, this Easter egg spawns two Forerunner weapons caches with powerful weapons if performed correctly.
Start up the mission Forerunner from Rally Point Delta. With the Ur-Didact's Cryptum in view, observe the Phantom that comes to drop off a small contingent of Covenant forces, including a Jackal Sniper.
Kill the Grunts and Jackal Sniper, but instead of heading towards either the left path or right path, continue forwards closer to the Cryptum and stand on the more brightly-painted section of the floor. Wait for a few seconds, turn around to see weapons caches carrying Forerunner weapons, and take a LightRifle.
Turn around again to face the Cryptum. There is a high-rise pillar on each side of it, with a wide hole at the top. Using the LightRifle, shoot at the pillars twice (but not in the same spot) and more than twice in the holes. It is highly recommended to use the LightRifle's scope for better aiming. After shooting the pillars, wait for several seconds.
If the Easter egg is done correctly, two weapons caches should spawn. The one on the left has three Incineration Cannons and two Boltshots, while the one on the right has three Binary Rifles and two Boltshots. Take the Incineration Cannons and Binary Rifles and feel free to use them throughout the rest of the mission; the Binary Rifle can be refilled later via drops from dead Knight Lancers, while the Incineration Cannon cannot be refilled outside the caches, so use the latter weapon wisely.