Infinite Energy Sword on Dawn
From Halopedia, the Halo wiki
The Infinite Energy Sword is an Energy Sword that can be acquired through a glitch on the Halo 4 campaign level Dawn. The weapon has unlimited ammo. This glitch requires at least three players to perform.
In order to obtain this weapon, start the level at Rally Point Alpha. Once you exit the Cryo chamber, begin to melee in order to delay the checkpoint from loading. Continue with the level in this way until you arrive in the room with the entrance to the elevator shaft. Once all the players are assembled in the room, kill the others so that they spawn with grenades. In order for them to spawn, you must transition from constantly meleeing to constantly jumping, which also delays the checkpoint but allows others to spawn on you. After they respawn, cease jumping and begin to melee again. The main player should stand near the elevator door, and the others should stand in a corner, bunched together. The main player should stand in such a way that they face away from the door, but still receive the prompt to pry open the door. Once all are in position, cease meleeing so that the checkpoint loads. Have the others kill themselves with grenades. Right before they respawn, open the door. If timed correctly, the other players will spawn inside the main player, and they will be able to shoot. The main player should then proceed up the shaft. Once you reach the top of the shaft, the other players must open fire on the Sangheili zealot and take down his shields as the main player grabs it to kill it. The Zealot should be killed by the others as you grab on to its head and throw it down the shaft. If timed correctly, the Zealot will drop an Energy Sword on the ground, which may be picked up. It has unlimited ammo, and does not degenerate after being used.