Spellbook of Mors Gotha
Mors Gotha's spellbook contained a number of copied spells - seemingly ones that the Guardian's minions frequently used to further their master's goals. Among them was a copy of the incantation which was used (most likely by Patterson) to erect the blackrock dome over Castle Britannia.
In their first meeting with Mors Gotha, the Avatar was able to obtain this book when the infamous swords-woman dropped it. After it was delivered to Nystul, the mage was able to analyze its content and create a method by which he could unweave the Guardian's enchantment, allowing the Avatar to shatter the black dome with the trumpeting of Praecor Loth's horn.