The Phantasmal Mask Jar represents their magnum opus. The lore book Fragments of the Fourth Seal (found in Chapter 3, hidden behind the waterfall in Thornwood) explains:
Some jars are specifically used to extract skills from phantasms, which can then be equipped by your hero. Types of Jars heroes lore 4 phantasmal mask jar
Each time the party rests at an inn, a random “resonance event” occurs — a spirit from the jar speaks in a riddle. Answering correctly grants permanent stat boosts; incorrectly triggers a difficult optional boss fight. The Phantasmal Mask Jar represents their magnum opus