The Thunderstorm skull is found the the level, [b] The Covenant [/b]. It turns all grunts into spec-op grunts (I forget if it increases the rank of brutes and jackals but I doubt it does).

[i][b]EDITOR'S NOTE:[/b] It gives a promotion to any unit that has ranks and can receive one, including Brutes.[/i]

You better be good at getting headshots because these grunts just won't die! I have to put a whole assault rifle clip into them if I don't aim for the head! Anyways, onto how to get it.

[b] Step 1 [/b]
After you deactivate the 1st tower, get into a hornet.

[b] Step 2 [/b]
Fly over to the 2nd tower that the Arbiter and the elites deactivated.

[b] Step 3 [/b]
Land on the 2nd tower.

[b] Step 4 [/b]
Climb up to the top of the big ramp.

[b] Step 5 [/b]
Pick up the skull.

Now you have the [b]Thunderstorm[/b] skull. Hope you enjoy wasting your ammo on one grunt and the 10g.

P.S. Congrats on the podcast!