In Assassin's Creed Valhalla Siege of Paris you can get abilities that grant you additional skills. This way, you won't have to rely only on the attacks you unlocked in the skill tree. On this page, you will learn the locations of Books of Knowledge in Amienois.
You can find the Goule Breath skill in Champlieu Ruins.
To get to the excavation site, you must enter a cave. This one is located under the boards - just destroy them and jump into the hole.
At the bottom, you will see a door that Elvior can open with the hidden blade.
Your have to destroy the cluttered wall. You can do this with the flammable jugs or with the special skill called the Incendiary Powder Trap.
A wooden shelf stands in the last corridor. Move it further into the room and then continue through the holes in the walls. Eventually, you will reach the ability.
You can find the Golden Flame skill at the raid location.
To get to the ability, you simply have to climb the tower and move a few shelves.