Cleanse the Earth Stone | Main story mode - Cleansing the Stones TES V: Skyrim - Dragonborn Guide

You should remember the Earth Stone - it stands right beside Raven Rock, on a small peninsula. Approach the construction and use Bend Will - it will crumble and a Lurker will appear nearby (it's possible that he will be joined by another one who's hunting nearby). Luckily you have to would-be constructors to help you - three of them are rather tough Redoran Guards. Together, you should be able to deal with them.

After dealing with the threat, touch the obelisk to learn a new skill - Bones of the Earth. After using, it protect the caster by absorbing 80% of physical damage for 30 seconds. As every stone-power, it has to be renewed after each use (in the location it comes from).