Complex sentence: God then ordered the Watchers to be imprisoned in the ground, and created the Great Flood (or the numerous Deluge myths) to rid Earth of the Nephilim and of the humans given knowledge by the Watchers.
Split into simpler sentences: God then ordered the Watchers to be imprisoned in the ground. He created the Great Flood (or the numerous Deluge myths) to rid Earth of the Nephilim and of the humans who had been given knowledge by the Watchers.