Complex sentence: Jaron manages to prove to the Regents that he is Jaron by telling them that a rock made of fool's gold is real gold, which was a last passphrase King Eckbert left in case anyone impersonated the prince.
Split into simpler sentences: Jaron manages to prove to the Regents that he is Jaron by telling them that a rock made of fool's gold is real gold. A last resort King Eckbert left in case anyone impersonated the prince.