How to Find the Champlieu Ruins Key
As you approach the location of Champlieu Ruins northeast of Paris, use your raven to scout the area and locate the entrance to the ruins. You’ll see a breakable wooden barrier with some dead guards around it. Shoot through the barrier and then drop down the hole to find a Hidden Ones door, which can only be opened by your Hidden Blade. Interact with the door to open it, then continue to the barricade at the southern side of the hall.
To reach the Book of Knowledge, pull the barricade toward you (northward) until it passes the hole in the wall to your right (west). You can now go through the hole in the wall further north on the western wall, dispersing the rats with melee attacks as you go, until they’re all inside the sewer grate. You can now loot the nearby pots and then slide under the hole in the wall to reach the Book of Knowledge, which provides the Goule Breath special ability.
Once you’ve looted the Book of Knowledge, slide back under the hole in the wall and take one of the fire pots. You may need to disperse the rats again first, since they’ll cause the fire pot to explode if they reach you while you’re holding it. If this happens don’t worry too much, as the fire pot will respawn. Take the fire pot back to the main hallway and use it to destroy the stone barricaded wall on the eastern side of the ruins. Beyond here you’ll find a hidden room containing the Champlieu Key.