Inside Koeglin mine solo dungeon. You can easily miss it because it is not on the way to the last boss. Go right just before the last boss and you will find it. Marked on the big map is the entrance and not the exact location of the shard (which is shown in a screenshot of the Koeglin mine map)
Enter the mine and proceed toward the back.
Turn right here.
Inside that side passageway you will find the collectible.
Exact location of the skyshard on Koeglin mine map