The Mason’s Channel, near the beach and in front of the Death Eater.
Light Elf Outpost. When exploring the island, you’ll have to circle around the path until you reach a dock by the cliff, right above the beach where you docked, as shown below:
Continue past that dock and you’ll see a chain holder ahead and a wall to the left that you can climb.
Use the chain to slide down and you’ll find a gate that you can lift that will serve as the shortcut back to the starting area. The mound is found by the broken ledge in front that gate, facing the waterfall at the far distance.