Complex sentence: While the opera was being played, large white snakes appeared, not afraid of people and not attacking them, seemingly watching the opera; the snakes were considered by locals as incarnations of Zhenwu, who came to watch the opera held in his honor.
Split into simpler sentences: While the opera was being performed, large, white snakes appeared; they were unafraid of people and did not attack them, seemingly watching the opera. The snakes were considered by locals as incarnations of Zhenwu, who came to watch the opera held in his honor.