In the level Arbiter on Legendary. Once you get to the first banshee section, fly immediately straight down all the way to the bottom of the map. In one of the cracks where the large cylinders connect to the station. There will be a circle of grunts dancing around a skull. When you pick up the skull; it will say Grunt Birthday Party on the screen. Now when you kill a enemy with a projectile weapon, they will blow up like a plasma grenade was stuck to them.
To get rid of this feature use the same method as the blind mode.