Skyshard can be found inside the temple ruins. Follow the main road and it will take you to this locationn. NPC Fatahala, that stands at the last crossroads before the Manor of Revelry, gives the quest “What the Heart Wants”.
Note: In order to reach this part of the map you need to complete the main quest “The Army of Meridia”. After “The Army of Meridia” you can get the quest that unlocks the entrance to this area of northern Hollow City.
Sand Kingdom Moons - Super Mario Odyssey Power Moons