Complex sentence: Like Thoth-Amon in Conan, he is powerless without his black ring and its destruction would cause his spell of living stone on Conan's family to be undone.
Split into simpler sentences: Like Thoth-Amon in Conan, he is powerless without his black ring. It was said that if his ring is destroyed, his spell of living stone on Conan's family would be undone.