The Teralith Helmet is a level 90 Melee tank head slot item, part of the Teralith 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 teralith helmet degrades to dust after 100,000 charges of combat, and cannot be repaired. Fighting in Raids does not degrade it at all. When wearing three or more pieces of the regular or Achto Teralith sets, or a combination of both, 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. Owning the teralith helmet also doubles the chance of getting the Achto Teralith Helmet as a drop from one of the Raids bosses. If you die in the Wilderness with the teralith helmet, it will drop 4,200,000 coins instead of the item.