Altara's Scepter is an extremely important Item in Ultima Underworld II. Its power is to cut the Guardian's eight lines of power from other worlds to Britannia, to weaken the Blackrock Dome enclosing Castle Britannia.
Altara told the Avatar about the scepter, but needed some more things to create it, having already altered a black pearl to give it special properties. The Avatar brought Altara the egg of a dread spider from Britannia and an amethyst rod from Talorus, so that she could create the scepter. With it, the Avatar deactivated all of the eight places of power of the Guardian on the other worlds to weaken the Blackrock Dome.