Moonhenge is an Ayleid Ruin that appears in Greenshade in The Elder Scrolls Online. This location is a landmark, and will count towards the "Discovering Greenshade" achievement. Inside this city, during the quest "Moonhenge's Tear," it is possible to travel to the location in the daedric plains, Isle of Torment.
Shut down what is being used to power the Daedric Portal.
On Isle of Torment: