Flaming knives on Osiris
From Halopedia, the Halo wiki
The flaming knives on Osiris is an Easter egg found on the Halo 5: Guardians mission Osiris. It can only be found on Legendary difficulty. If the Easter egg is performed correctly, the game activates a special checkpoint.
There are seven knives hidden around the mission. Damaging these knives in any way (ranged weapons, melee, or grenades) causes them to light on fire.
For the first knife's location, stick to the right wall after the first cutscene of the mission. Jump on top of the wreckage, and then jump onto the rocks right next to it. Here you will find the first knife. Once lit, the first knife is activated, which turns almost all Forerunner weapons in weapon caches into UNSC weaponry.
The second knife can be located in the first large area filled with Crawlers. Before you go through the narrow path to the next area, take a sharp left. Along that wall, there is more ship wreckage. The second knife is located right next to it on the small rock outcropping.
The third may be found in the next area. After you clear the area of enemies, jump on top of a rock that is jutting upwards in front of the door. From there, you can shoot at the knife, which is located in the back of the canyon and just right of the large ice overhang.
The fourth is in a canyon in the next area. After you exit the Forerunner tunnel (The AA cannon should be close by), turn left and go to the ledge. Zoom in on the cliff directly across from you, and you can find the knife sticking out of the snow right above the drop-off.
The fifth isn't as easy to find. Inside the tall, narrow canyon (There are 2 crashed Banshees on the overhang above you) go to the cliff where the path turns back to the left. Jump up onto a small rock just right of the ledge, and look back towards a large group of icicles on the same level as the closest Banshee. In between the gap of icicles, there is a large, dark area. This is where it is located.
The sixth is found in the large area with the Splinter Turrets. After all enemies are eliminated, go to the door you are supposed to enter. Before you enter, take a sharp right and stick to the wall and clamber over a small ledge. Follow the wall until you get to the cliff, and the knife is located on a small outcropping a short distance from there.
The seventh and final knife is located in the center of the large Forerunner room with Knights. Kill all the Prometheans, and then stand on top of the center pedestal. From here, shoot the knife which is located on the pedestal when looking towards the deployable cover shields.
If you activated all the knives, a checkpoint will occur. The room will start to rumble, and then the door explodes. During this time, your weapons will be replaced with a Prophets' Bane and a Plasma Pistol. You also gain active camouflage as an oversized Promethean Knight and the Warden Eternal come through the door located in front. The Warden is much taller than he normally is, and seems to have a little more health.