Follow the southwesternmost path in this area. The entrance to the dungeon is approachable from the road. It’s a tiny wooden door, between the rocks, close to some medium sized mushrooms. The dungeon quest called “A Hidden Harvest” is given by Halinjirr just before the entrance. Although it won’t lead you toward the shard at the start, the additional experience is always nice.
If you are in the dungeon for the sole purpose of finding the skyshard, head to the northeastern part. On the ground level of the main central room, there is a waterfall that hides a passage behind it. Go through it, and get through the small tunnel.
The golden rays in this case are the sun’s radiance, just on top of the platform where the cliff striders dine. According to the bones, you were the lucky one.
Sand Kingdom Moons - Super Mario Odyssey Power Moons