Found in dungeon delve Fearfangs Cavern, west of town Dragonstar, northwestern Craglorn.
Soloable: Yes (there is a veteran rank 13 boss Sepilisk near the shard. It is highly possible you need at least his veteran rank in order to collect the shard without alarming Sepilisk).
On your way to the shard you can find many groups of enemies with their snake friends. These groups are found in small rooms full of bookcases. Who knew snakes like to read. Largest of them all – boss Sepilisk – guards the shard in central cave room. You can, however, go around Sepilisk and collect the shard without too much trouble. Be careful though, because when I did it I was VR14, so it might be more difficult for lower levels to bypass the boss without alarming him.
Fearfangs Cavern Entrance…
…and its map location.
Sand Kingdom Moons - Super Mario Odyssey Power Moons