In the Kamiumi-no-dan (the Chapter of Bearing Gods) of Kojiki, when Izanagi cut the god of fire Kagutsuchi, which had caused death to his wife Izanami, on the neck with his sword Totsuka no tsurugi, Iwatsutsunoo was born from the blood on the sword which dropped on the rock.
古事記の神産みの段でイザナギが十束剣で、妻のイザナミの死因となった火神カグツチの首を斬ったとき、その剣の先についた血が岩について化生した神である。