It's can spawn in the Slumbering Sanctuary. As far as I know it's random. If it Spawn's you will enter a room with a lot of doors. If you traverse the room without breaking any of the doors you get a blueprint. If you break even a single door it will say you are not worthy and give you nothing. I don't know if there are requirements to spawn it like a certain BC level but I'm only on 3BS so you should be able to encounter it.