Halo 5 Knife Easter Egg

To achieve this easter egg, you have to shoot 8 knives found in the first mission to unlock a type of energy sword called Prophet’s Bane and fight a giant Warden Eternal and a Giant Knight:
Here is the full tutorial by ReadyUpLive: HALO 5 | KNIFE EASTER EGG | GIANT ENEMIES, PROPHETS' BANE (Halo 5 Guardians) - YouTube