The Tempest Cowl is a level 90 Ranged tank head slot item, part of the Tempest equipment set. It can be purchased for 8500 Teci (rewarded for completing Raids) from the Armoursmith in Mazcab. Besides Daemonheim-only armour, it offers the best defensive bonuses for any item in its slot, tied with other Raids equipment.
The tempest cowl degrades to dust after 100,000 charges of combat, and cannot be repaired. Fighting in Raids does not degrade it at all. When wearing the full regular or Achto Tempest set, or a mix of the two, there is a 2% chance per piece that all of your defensive abilities will have their cooldowns reset. When this happens, there is a 1 minute cooldown before they can be reset again. Wearing the tempest cowl also doubles the chance of getting the Achto Tempest Cowl as a drop from one of the Raids bosses. If you die in the Wilderness with the tempest cowl, it will drop 4,200,000 coins instead of the item.