The Primeval robe legs are a level 90 Magic tank legs slot item, part of the Primeval equipment set. They can be purchased for 10000 Teci (rewarded for completing Raids) from the Armoursmith in Mazcab. Besides Daemonheim-only armour, they offer the best defensive bonuses for any item in their slot, tied with other Raids equipment.
The primeval robe legs degrade to dust after 100,000 charges of combat, and cannot be repaired. Fighting in Raids does not degrade them at all. When wearing the full regular or Achto Primeval 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. Owning the primeval robe legs also doubles the chance of getting the Achto Primeval robe legs as a drop from one of the Raids bosses. If you die in the Wilderness with the primeval robe legs, it will drop 4,800,000 coins instead of the item.