Go inside the large West Portal watchtower to find this skyshard. This is the portal that you need to close for the quest “Crossing the Chasm” – the first quest you receive upon completion of the main quest “The Army of Meridia”. Shard’s journal tip kinda tells the whole story.
Note: In order to reach this part of the map you need to complete the main quest “The Army of Meridia“. Next quest unlocks the entrance to this area in northern Hollow City.