Complex sentence: The Spaniards called the island "Isla del Fuego" or "Island of Fire," because the island gave off an eerie glow, which came from the great swarms of fireflies that harbored in the numerous molave trees on the island.
Split into simpler sentences: The Spaniards called the island "Isla del Fuego" or "Island of Fire," because the island gave off an eerie glow. This glow came from the great swarms of fireflies that harbored in the numerous molave trees on the island.