Thunderstorm (achievement)

Thunderstorm is a Halo 3 Campaign Achievement. It is earned when the player finds the Thunderstorm Skull. This Achievement gives 10 Gamerpoints to the player.

It is represented by a blue circle with a skull in the middle. The skull has a thunderbolt on its forehead.

Effect: "Field promotions for everyone." All enemy units are promoted one full rank (Grunt Minor to Major).

Found on the level "The Covenant". After you take command of one of the Hornets, Fly toward the second force field generator (the one the Arbiter disabled) and land. the Thunderstorm Skull is waiting at the very top, at the end of the ramp that extends over the sea.