"Field promotions for everyone!"
The skull, Thunderstorm is found on the level The Covenant. It is found in the second tower that The Arbiter was sent to shut down and succeeded. It can only be accessed via Hornet during the air fight. You must bank right as soon as you take off and you'll see a set up very similar to the one you would be landing if you went straight to the third tower. Land on that area and go around to the middle area, which rises up like a stair case, or just landed their in the first place. It overlooks the area where everyone is having their dog fight, but the best part is the skull tucked right there at the end of the platform. Pick it up for a smooth 10 and a new skull to use in Campaign.
The effects of this skull are simple, it promotes all enemy units. So instead of facing blue armored Brutes, you'll now face up against the upgraded version which are gold armored Brutes. It basically just raises the difficulty a little bit, but nothing noticeable while playing on Normal.
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