The Skull of Power is inside the Hall of Bones and legend has it that the skull glows with the power of those buried here. It's full purpose is not explained but it can mystically energise items placed inside its jaws and the heroes use it to recharge their depleted totems. When the heroes climb inside the skull they are teleported outside the hall to the nearby mountainside. The skull is destroyed along with the hall in a battle between Venger and the spirit warriors.
Bobby seems to think that the recharged totems will last for three hundred years but it is unclear where he got that number from and could just be guessing or joking that they need not worry about recharging them further in their lifetimes.