Instructions
Exit Mitsurugi's shrine after you have defeated him. This is the area adjacent to Datara's shrine, and you must defeat Mitsurugi and Sugagaki before you can confront Datara.
Travel along the dirt path heading north from Mitsurugi's shrine. Continue along this path until you reach a mountain range near a large body of water. Travel to the west along the water's edge until you find a waterfall. Enter the waterfall to find a hidden cavern.
Walk south from the cavern's entrance to find a bridge-like structure. Traverse all the way down this bridge to the southern portion of the cavern -- this is quite a long walk with numerous enemies, but they are fairly weak.
Continue traveling along the linear path until you reach a warp tile -- a small, golden circle. Step onto it to be warped to the inner sanctum of Sugagaki's shrine.