Complex sentence: When Conan's Star Metal sword is close enough to them it broke the spell that disguised them and revealed their true form to be Serpent Men; when Star Metal made contact with the Serpent Men it banishes them to the Abyss with Set.
Split into simpler sentences: When Conan's Star Metal sword is close enough to them it broke the spell that disguised them and revealed their true form to be Serpent-Men. When Star Metal made contact with the Serpent-Men it banished them to the Abyss with Set.