How do I open The Vault in Fociaugh Hollow?
In FFXV There is a cave You Enter called: Fociaugh Hollow, and Prompto gets taken by a Nagi. She asks you "Where is it?" We don't know, but we Do however find a vault. How do we open the vault?
The door you are asking about is apart of the series of side quests known as Menace Beneath Lucis. Unfortunately, base on the dialog you provided, you cannot open the door at this time.
This door cannot be unlock until you completed the following tasks:
- Complete the story and reach the post game
- Complete the dungeons: Costlemark Tower, Crestholm Channels, Balouve Mines, and Daurell Caverns
Once you met the required criteria, make your way over to the Hunter's Headquarters located southeast of the Vesperpool, and speak to Ezma. She will provide you the side quests and the Sealbreaker's Key to open the door(s).