Complex sentence: It was also thought that a cockatrice would die instantly upon hearing a rooster crow, and according to legend, having a cockatrice look itself in a mirror is one of the few sure-fire ways to kill it.
Split into simpler sentences: It was also thought that a cockatrice would die instantly upon hearing a rooster crow. According to legend, having a cockatrice look itself in a mirror is one of the few sure-fire ways to kill it.