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The Fourth Part of the Will is the teritary villain in The Keys To the Kingdom series.
It takes the form of a snake wrapped around the sword that is the Fourth Key, wielded by Sir Thursday. The Fourth Part of the Will is able to free itself, but first Sir Thursday must be distracted sufficiently; this was accomplished by invoking his wrath. After being freed, it is obsessed with bringing Thursday to justice, and is barely subdued by Arthur. When Arthur goes to meet the Piper, the Fourth Part of the Will joins him, and contrary to Arthur's wishes, spits corrosive acid into the Piper's mouth. The Will assures Arthur that as this acid was not "poison", it did not disobey his command to refrain from poisonously biting the Piper (being able to choose whether or not to be poisonous, and if so, to choose its type of poison). During Arthur's meeting with the Piper, the Piper notes that the Fourth Part of the Will speaks in a tone that is familiar to him. The Fourth Part of the Will embodies Justice.
At the beginning of Lady Friday the Fourth Part of the Will has joined Dame Primus. Dame Primus thereafter begins to pay attention to justice and to exhibit snakelike features. She orders all the Piper's Children in demesnes ruled by Arthur to be killed, as the Piper is their enemy; Arthur quickly overturns this order. The appearance of her snakelike fangs and forked tongue, possibly provoked by a moment of passion, is the second instance of suspicious behavior displayed by Dame Primus (the first being refusing a call from Leaf, who was then trying to help Arthur).
Dawn, Noon and Dusk