One legend in the Sangoku buppō denzū engi 三國佛法傳通緣起 (1311) by the monk Gyōnen 凝然 (1240–1321) states that Śubhakarasiṃha (637–735) came 728–729 to Japan to teach and spread the secret teachings mikkyō 密教 of esoteric Buddhism. He built a small lodge in the south-west of the Tōdaiji 東大寺 that was not yet finished at that time. On the same place Kūkai 空海 (774–835) later founded the Shingon-in 眞言院. When Śubhakarasiṃha recognized that he could not find anybody who understood his teachings, he built a pagoda tō 塔 in the east of the Kumedera 久米寺 in Nara 奈良 and hid a copy of the Dainichi kyō 大日經 under the central pillar 心柱. He made a wish that one day the right person will find it. And indeed it was found by Kūkai.(Rambelli, Fabio. Texts, Talismans, and Jewels: the Reikiki and the performativity of sacred texts in medieval Japan. In Payne and Leighton (2006: 70).

Reference:

Payne, Richard, and Taigen Daniel Leighton. 2006. Discourse and Ideology in Medieval Japanese Buddhism. Taylor & Francis.