The Beast Stone can be found right next to the Temple of Miraak - just head a bit to the east (a pillar of light should lead you). Once there, use Bend Will and wait for the construction to collapse. A Lurker will appear somewhere in the area and the nearby Rieklings will run away to a safe distance. If you have problems defeating the beast, force it to attack them - they shouldn't have any problems with it.
Touch the obelisk in the middle of the destroyed construction to learn a new skill - Conjure Werebear (it allows you to summon a powerful ally). Look out, as you can only use it once (afterwards you will have to renew it by touching the obelisk again).