"Cleanse the Beast Stone" is an objective in the main quest "Cleansing the Stones" in Elder Scrolls V: Skyrim - Dragonborn.
The Beast Stone is found to the East of the Temple of Miraak. Head down the slope to the marker to find the Beast Stone.
Unlike most of the All Maker Stones, this stone has a Cultist watching over it. You will need to deal with the Cultist to make things easier for the next step.
Cleanse the Beast Stone using the Bend Will shout to faces its guardian Lurker