The Tome of Ulurslef

This is a book introduced in The Patron's Promise (One-shot). It's attuned to the eldritch entity Ulurslef and is able to banish, summon, and otherwise control them. After Ulurslef's team of warlocks fought back with the book and banished the entity, the book was left in the abandoned Zendarru Nightfall Hold. Maria found it when the WoDaD party investigated the hold - the book tried to enchant her to read it, but due to her blindness, she was unable. Later, in Zendarru, Maria and Chi Pin took the book out and started to summon Ulurslef before they managed to stop each other. Still, Ulurslef made a deal with Chi Pin to summon them back into the material plane, and when they were back they managed to effectively destroy the book's power over them... or so it seems.