Complex sentence: As the hole gets bigger, Arthur decides to use the Fourth Key, even if it turns him into a Denizen, to fix the hole; one Key is not enough, so the Will tells him to imagine that he is holding all the Keys.
Split into simpler sentences: As the hole gets bigger, Arthur decides to use the Fourth Key, even if it turns him into a Denizen, to fix the hole. One Key is not enough, so the Will tells him to imagine that he is holding all the Keys.