Found inside Portdun Watch solo dungeon. Dungeon’s entrance can be found inside a tower close to the south wall of Firebrand Keep. Marked on Stormhaven map is the entrance into the dungeon and in the screenshots you can find the exact location of the collectible.
Enter Portdun Watch
As you reach the last boss of the dungeon (seen on the left in the screenshot above) you will find the skyshard to your right.
Exact location of the shard