The Seventh part of the Will appears an apple tree. It is able to communicate with Arthur with restrictions and its capabilities to do so improve as Arthur gets closer. The Seventh part of the Will is locked up in a cage, that the Will explains brings death to all that touch it and can only the Seventh Key or the Mariner's Harpoon can open (which subsequently kills the Mariner).
It represents the virtue of Hope and is likely a reference to the tree of knowladge in the Garden of Eden. It uses its roots and branches to snare Lord Sunday giving Arthur time to claim the Seventh Key.