Complex sentence: The former Witch of the Waste discovers it and promptly destroys the bug by tossing it into Calcifer, who then gets sick after eating, rendering him unable to protect the castle from being discovered due to his weakened state.
Split into simpler sentences: The former Witch of the Waste discovers it and promptly destroys the bug by tossing it into Calcifer. Unfortunately, Calcifer gets sick after eating the bug, rendering him unable to protect the castle from being discovered due to his weakened state.