Complex sentence: At the auspicious time the High Priest of Shiva will invite the king to take a ceremonial bath, for which purpose the king will change into a white robe, symbolic of purity (similar to a monk's Kasaya).
Split into simpler sentences: At the auspicious time the High Priest of Shiva will invite the king to take a ceremonial bath. The king will change into a white robe, symbolic of purity (similar to a monk's Kasaya).