Complex sentence: The Spanish conquerors were about to lose, when suddenly the sky went dark and Saint James the Greater appeared holding a fiery Holy Cross, causing the Indians to immediately concede, after which the city of Santiago de Querétaro was founded on July 25.
Split into simpler sentences: The Spanish conquerors were about to lose, when suddenly, the sky went dark and out of it came Saint James the Great and a fiery Holy Cross. The local Indians immediately accepted defeat, and so the city of Santiago (Saint James) de Querétaro was founded on July 25.