To enter Ilthag’s Undertower delve (which contains the skyshard) you will have to
find a basement door in a ruined tower just east of Valley of Scars Wayshrine (Northeast of a nearby lake).
Soloable: No (there are a lot of groups between its location and delve’s entrance/exit. There is also a whole group standing near the shard).
Once you enter the delve, jump into a gap found on floor. This takes you to the lower part of the dungeon. There are large numbers of enemy groups found here. Run/fight/stealth your way to the northern part. There are stairs you can use to climb up out of this lower part of the cave. When you go through a small passage that connects the lower and higher part of the cave you can spot the shard easily. It is on a small wooden platform. It is also guarded by a large group of mobs.
Sand Kingdom Moons - Super Mario Odyssey Power Moons