Cloud

Were you looking for Fog, another skull with the same effect?

Cloud is a gold skull found in Halo: Reach and Halo 4. It replaced the Fog skull found in Halo 3, Halo Wars and Halo: Combat Evolved Anniversary.

Effect
The motion tracker is removed from the HUD, leaving players oblivious to potential threats.

Trivia

 * The cloud skull has the same symbol, and effect, as the Fog skull.