A spellbook belonging to the military leader Mors Gotha.
It somehow got sent to Britannia prior to Ultima Underworld II by the Guardian in order for mayor Patterson to use it to initiate the spell that, with the Guardian's help, would create the Blackrock Dome. Afterwards, it was returned to its owner.
Much later, after the Avatar fought Mors Gotha on Killorn Keep, she lost the book during her escape through a Moongate and it fell into the Avatar's hands, who gave it to Nystul for analysis. Nystul managed to use it together with the Horn of Praecor Loth to shatter the dome, freeing Castle Britannia from its prison.