The Fifth Part of the Will appears as a large, half-reptile, half-bat monster or Wyvern, worshipped and feared by the Winged Servants of the Night. Arthur finds it imprisoned by Lady Friday  in the "Inner Darkness" of the cave system the Winged Servants of the Night use as their eyrie, on the Top Shelf of The Middle House. Known to the Servants as the "Beast", the Fifth Part of the Will is a pleasant and agreeable creature, unlike the other parts. It mentions that the "Key" Friday left is not the real Key, but a trap, and that she has not legally abdicated her rule of the Middle House. After being released, the Will accompanies Arthur, Suzy , Fred , Banneret Ugham, and a host of Dawn's Gilded Youths to Lady Friday's Scriptorium, where Friday claimed to have left the Fifth Key.

There, they find that a battle between the Piper and Superior Saturday's forces has left Saturday's Dusk dead, and the Pipers Children unconscious. Despite Arthur's and the Will's warning of a possible trap, the Piper orders Ugham to take the fake Key from its pedestal, activating the trap and breaching the Void of Nothing. This kills Ugham and quickly consumes the Scriptorium and most of the Top Shelf. The Piper uses his Pipes to enter the Improbable Stair, in order to escape, while the Will manages to save Arthur and his allies with a sweep of its tail. The Will then tells Arthur to use the Keys to close the rift into the Void of Nothing; after summoning all four Keys to help him, he successfully closes the Void. Using the Keys, Arthur takes the Improbable Stair to Monday's Dayroom, where Sneezer sends them to Lady Friday's sanctuary in the Secondary Realms. There, with the aid of the Will, Arthur defeats Friday.

After defeating Lady Friday in her sanctuary, Arthur gives the first four Keys to the Fifth Part, to return them to Dame Primus. Before leaving, the Will apologizes for the behavior of the other Parts of the Will, speculating that Dame Primus will be easier to work with once it has joined her; this is later shown to be incorrect, as she acts even more rebellious than before. It represents the virtue of Temperance.

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