The Mysterious Runed Rock was an entry in the Agility level-up guide, which appeared on or before the 10 October 2016. It required level 95, and even though it is listed as an agility course, it has the icon of a yellow "balance-on" type shortcut.
Currently, it is not known what the runed rock is - be it a well-hidden easter egg, a teaser for a future update, or merely a mistake.
It was removed from the guide on 16 January 2017 or earlier as a hidden update.