Just go west, right after you unlock the first door using Royal Knife.
Right after Sorey obtain the power of Armatization. Move forward a little and you will find it.
Continue a little bit from the monolith above and you will find it. It’s on the same room where you need to lit 2 torches.
The area can be accessed once you reach certain point.
Go west at the start of the dungeon (dead end path).
Located in one of the prisons on Vivia Aqueduct.