Scattered around the island are six stones similar to those that you've seen. Of those stones, you've cleansed one, seen at least the one in Raven Rock and found the Tree Stone at the temple of Miraak. The Tree Stone seems to be a hopeless case but Storn thinks that if you can cleanse the remaining four stones then you might weaken Miraak's hold over the people of Solstheim.
The four remaining stones will be marked on your map. Clearly the easiest one is in Raven Rock but there are a few more scattered around the island. There is one NW Skaal village, another north of Raven Rock and the last is between the Skaal village and Noleth's home.
Cleansing these stones will be the same as what you did at the Wind Stone. Go to the stone, shout at it and a Lurker will appear. As you cleanse the stones, you'll end up fighting more Lurkers as well as Cultists. Over the course of the four stones, however, they should ramp up in difficulty slowly. Take your time and gauge how you do at each stone before moving to the next. When you've taken care of all four, you can return to talk to Storn or just continue on with the quest, The Path of Knowledge. When you complete that quest, you will return to Storn anyway.
