Helm of the Rock is a magical item which grants the wearer, who must be a fighter, 25% resistance to fire, cold, electricty, acid and poison. The helmet has a weight of 2 lbs and it does not provide protection against critical hit. This item appears in Baldur's Gate II: Throne of Bhaal and this item will probably appear in Baldur's Gate II: Enhanced Edition. Cespenar can upgrade the helm for you, which requires 5000 gold.
This helm grants a warrior immunities far beyond mere physical protections. It is obvious there once used to be a set of horns protruding from the metal cap, though the horns and any magic associated with them have long since disappeared.
When the horns are added to the helmet, it can be used thrice per day to cast Aura of Command, which forces enemies to make save at -2 or flee in terror. The effect of the spell can be compared to Cloak of Fear.