There is a large ruin just west from the Urshilaku Camp Wayshrine. Go up the stairs, and find a small passage that leads to the eastern side. This passage has a lot of pillars on the sides. It leads to the metal, round dungeon door. Go through them to enter the dungeon.
Head northeast from the dungeon’s entrance. There is a small map arrow pointing toward the second area. Proceed there, and inside the next zone, you’ll reach a small access corridor that leads to the southern side. It is found on the ground level, close to the water. Follow the path to the southernmost room. Defeat the Worm Cults who pray to the shard to acquire its power.
Sand Kingdom Moons - Super Mario Odyssey Power Moons