Complex sentence: Miroku is to destroy a yōkai who threatens to cast the world into eternal night, at which the first sign of the yōkai's appearance for the first time in fifty years is a full moon shining for many nights.
Split into simpler sentences: Miroku is to destroy a yōkai who threatens to cast the world into eternal night. The first sign of the yōkai's appearance in fifty years is a full moon shining for many nights.