When Kobo Daishi Kukai visited Yokoku-ji, tradition relates that he struck a rock with his ritual vajra to cause water to gush forth; a mother monkey and her child suffering from eye disease washed their eyes in the water and were instantly healed. Kukai named the water "Okozui" (Vajra Water), and it became renowned across Japan as a healing spring for eye ailments. Since then, the temple has been venerated as the "Kannon of the Eyes" and a sacred site of Shingon esoteric Buddhism associated with Kukai, drawing devotees praying for eye health to this day.