The Blacksmith Achievement in A Plague Tale: Innocence
Find Rodric's forge
How to unlock The Blacksmith
During Chapter 16, Amicia, Hugo and their companion Rodric will reach a point where they encounter two civilians being captured by three soldiers near a big cart. Upon killing the soldiers to free the civilians, the trio will discover that their route is blocked by archers. Before interacting with the cart to use it as a cover to proceed, make a U-turn to locate an alleyway blocked by a gate locked from the inside. The very end of this alleyway leads to Rodric's forge. To open the locked gate, enter the plague-infested corridor to the left of the previous alleyway, and look right to locate an opening which allows Amicia to break the lock on the gate with her sling. The achievement unlocks after Rodric has seen what is left of his forge.
Consult the following video if you have any trouble. All credit goes to its maker, Trophygamers.