The weapon assassination glitch occurs only in Halo: Reach. This allows a player to assassinate with their secondary weapon. To do this, the player must have an energy sword and a secondary weapon of choice. To perform the glitch, assassinate another player by holding down the melee button and switch weapon button at the same time. (NOT Swap Weapon!) If done correctly, your secondary weapon will be used as the knife in the animation.
This glitch may/not work on match-making, custom games or other game types. So feel free to test it out!