Blackrock Dome

The Blackrock Dome is a magical construct created around Castle Britannia by the Guardian. It appears during Ultima Underworld II.

Description
The Blackrock Dome appeared in the early morning hours of the day following the Feast of Rebuilding, imprisoning all revelers who had taken their rest within Lord British's castle. A massive structure of magically summoned blackrock, the dome dampened the magical abilities of all trapped within it, making the task of dispelling it difficult.

Due to the crude working of its creation, however, the dome created a series of magical "echos" throughout its interior, producing physical copies of itself not only within the depths of the castle but upon other worlds whose sources of power the Guardian had drawn upon to craft the spell. The Avatar was able to discover a large multi-faceted blackrock jewel within the lowest regions of the keep's sewers which had manifested within at the time of the dome's creation, and by stepping through its faces, the hero was able to traverse the planes to other realms which the Red Titan had touched.

On these other worlds, the Avatar sought out additional physical reverberations left by the spell, which took the form of a number of black gems of varying shapes. By fusing this gems with the larger jewel, the hero was able to weaken the magic which held the dome in place. After using a magical scepter to cut the lines of power from which the Guardian drew strength and discovering the spellbook from which the initial incantation was cast, the Avatar was able to assist Nystul in unworking the spell which held the dome together, rending it fragile enough that the hero was able to sunder it by sounding the Horn of Praecor Loth with the powers of a bound Air Djinn, freeing the inhabitants of the castle and shattering the jewel within which had formed a gate to other lands.

Trivia

 * It is most probable that Patterson was the individual who performed the incantation to summon the dome, although this is not explicitly stated in the course of the game.