Mercy Stroke is a dagger weapon skill.
Mercy Stroke was a Marauder and Warrior ability available after reaching level 26. The ability delivered an attack with a potency of 200. It could only be executed when target's HP is below 20% and should the player use the ability to deliver a killing blow, they would have 20% of their maximum HP restored. The ability also had an additional effect trait known as Enhanced Mercy Stroke available at level 40, which lowered the ability's cool down to 40 seconds.
At the launch of the Stormblood expansion, the ability and the trait were both removed from the Marauder's skillset and the game entirely.
Mercy Strike is an active ability learned by Thief (5★, level 78). It deals physical damage (2x) to one enemy and decreases their ATK (30%) for 3 turns for 17 MP.