This is so retarded. There is no legend of an imprisoned demon. This is all made up for internet points.

The stone is believed to be the transformed corpse of Tamamo-no-Mae, a beautiful woman who was exposed as a nine-tailed fox working for an evil daimyō plotting to kill Emperor Konoe and take his throne. According to the otogi-zōshi, when the nine-tailed fox was killed by the famous warrior Miura-no-suke, its body became the Sessho-seki. Later, a Buddhist priest called Genno stopped for a rest near the stone and was threatened by the spirit of Tamamo-no-Mae. Genno performed exorcism rituals and begged the spirit to consider her salvation. Tamamo-no-Mae relented and swore never to haunt the stone again.

The legend is that there USED to be a already-dead demon spirit haunting the rock... and the spirit has already been dealt with long ago.

Edit: and of course Snopes says it is true, so of course it must be false. (Went and read the snopes article - they quote the same exact wiki page as I did but did not attribute the quote, ie they plagiarized... and they stopped the quote right before the part about the Buddist priest so that the quote seemed to support their clickbait BS)

Heaven impregnated Earth and left a stone egg that hatched Monkey, who eventually became the Great Sage Equal of Heaven. After some shenanigans, he'll help Tripitika on the journey West to find the holy scriptures.