The cult of Acala was first brought to Japan by the esoteric master Kūkai, the founder of the Shingon school, and his successors, where it developed as part of the growing popularity of rituals for the protection of the state. While Acala was at first simply regarded as the "primus inter pares" among the five wisdom kings, he gradually became a focus of worship in his own right, subsuming characteristics of the other four "vidyarāja"s (who came to be perceived as emanating from him), and became installed as the main deity ("honzon") at many temples and outdoor shrines.