This mission can be activated by speaking to Adril Arano in Raven Rock - just ask him about Miraak. Some characters living in the city might initiate this mission as well, if only you ask them. Regardless of how you begin the mission, your task will be examining the Earth Stone, built on a peninsula beside the village (screen above). Go there and you should be approached by Neloth, who will tell you that mysterious powers have been in control of the village for some time now. They order ordinary people to get out of their houses and build strange monuments - just like the one you're standing before right now.
A certain clue in this madness might be the mysterious construction known as the Temple of Miraak. It seems that both events - the assassination attempt and the strange behavior of the villagers - are somehow connected. You should head to the distant construction and search it for further clues.