"Cleanse the Sun Stone" is an objective in the main quest "Cleansing the Stones" in Elder Scrolls V: Skyrim - Dragonborn.
The Sun Stone is found to the North of Tel Mithryn.
Approach the Sun Stone. You will find a group of Reavers are working on this Stone's temple. They will not turn hostile at any point during the conflict or after. they will actually help with what comes next. Approach the Stone and use the Bend Will shout.
Cleansing the stone will unleash another Lurker. Be ready for the attack. Thankfully, you will have the aid of the nearby Reavers that were working on the temple.