This collectible is inside the public dungeon Nisin Cave. Entrance to Nisin Cave is just north of three-ponds lake. There is a camp of hostile creatures in front of the entrance. If you plan to solo the dungeon, you should be aware of a couple of things. You really do not want to die inside. If you die, it means you will be teleported to a wayshrine your faction holds. Skyshard is hidden inside the last Nisin Cave room. There are wisps inside the cave that patrol the area. Try not to pull them while cleaning the way towards the collectible. In my case, there was one strong creature in a tunnel between two largest Nisin Cave rooms. I was able to pass him by without alerting him. If you are up for a challenge, try defeating him after collecting the skyshard. It is possible to split apart a group of mobs by pulling one of the members, with a range spell.
Camp outside Nisin Cave.
Nisin Cave entrance.
Nisin Cave entrance map location.