The Cowbell skull makes all explosions bigger. It's the third silver skull.
The Cowbell skull is fairly far into the level The Ark. Start by destroying the Scarab. Now look into the desert where you battled the Scarab and you will see a fairly large structure (it’s the building you see if you are at the bottom of the ramp you are supposed to go up, and turn to look in the opposite direction). Go in and grab a gravity lift. Now that you got the gravity lift go into the building with the cartographer inside. Pass through the rooms untill you reach a hallway with 2 ramps (after a room with a lot of explosive crates). The skull is at the top of the room. When you turn to the second ramp stop and look up. Above you are 4 platforms. The skull is on the highest platform. Use the gravity lift to get to the top.
Warning nothing is easy dont start the level halfway into the level. For example don't start from checkpoint Alpha (or whatever the name of the checkpoint is) or the skull wont be there.
In order to get to the top I recommend to first look around. In the walls there are indents (little slits running horizontally across the wall ). Get near the place where you want to deploy the Grav lift. Now go near the wall. Jump and press X. If you do this method correctly the gravity lift will get stuck in the wall. If it gets stuck too high just grenade jump onto the lift. This is a much less painful method.
The skull is a great boost to your grenade jump just a bit more painful for master cheif. You can also combine this with the Grunt's bithday party skull for a very special party, one a grunt will never forget and if it's their last party you want them to go out with a bang. Its also a good idea to throw a grenade into a group of flood to exterminate the imediately.
Editor's Note: Even if you deploy the gravlift normally on the floor of the ramp, you don't actually need to reach the top ledge and stand on it to retrieve the skull; just hold RB near the apex of your jump and you've got a good chance of grabbing it.