At the end of the level the Arbitor when get a Banshee go to the Bottom of the structure. Look around and you will see something like a bridge that looks like it is holding the structure up. Check under the bridge and you will find 2 ramps on one side will be a number of grunts dancing around a skull. Pick up the skull.You must do this on Legendary.

The skull GRUNT'S B-DAY use is headshots turn into Plasma Grenade explosions. You must first take down an Elite's shield for this to work on one. You can also give a headshot to a dead body and it will still explode. This skull doesn't affect Hunters or Drones. Note: you can combine this with the Sputnik skull, this will make the explosions really powerful and amusing. (it is also A LOT easier to obtain with the Sputnik skull)