Merciful Achievement in A Plague Tale: Innocence
Save the dying
How to unlock Merciful
During Chapter 12, as Amicia and Lucas make their way back to the now-abandoned de Rune estate, they will encounter two wounded English solders, one sitting on the ground against a wall and the other one leaning against a nearby cart. Do not destroy the light sources protecting the two men or rats would devour them. If Amicia explains to Lucas that the soldiers were dead anyway and she had enough blood on her hands as they progress to the next area, that means the soldiers' lives are successfully spared and the achievement would unlock at this point.
Consult the following video if you have any trouble. All credit goes to its maker, NonstopTrophies.