Helm of Tohan is an official plugin that includes a new quest to find the titular legendary artifact. The quest can be started by asking for rumors around Dagon Fel, which will direct you to the Daedric ruin of Onnissiralis. Outside the ruin are two warring brothers, one of whom will accompany you as you enter the ruins to find the artifact. It is not an Artifact in the usual sense in TES games, as it does not come pre-enchanted.