I am a GM for both DnD 5e and Pathfinder 1e. I was thinking how some creatures in stories only die with a blow to a very specific spot and all hits anywhere else are completely nullified, and how common of a trope this is. I don't plan on using a creature like this but was brainstorming how would such a thing be implemented in game and I came up with two ideas:

• They only take damage on critical hits (this is not a fun idea that I would ever implement, but it fits the concept)

• Getting inspiration from werewolves who are generally only harmed by silver but in DnD have partial resistance only to other things. This is fair and good, but would still not fully implement the concept.

How do you implement a mechanic that would translate the idea of "this creature will die if hit in X spot and is invulnerable everywhere else"?