In considering how to handle ‘fatal blows’, specifically how to cause a particular death blow, I wanted to find a way to target specific body parts. I can see cases where a particular killing blow would be useful (staking a vampire or decapitating it both come to mind).
I believe I’ve found a fairly simple way of handling this, working in two parts.
If you can apply the coup de grace action to a creature (it is helpless and you take a full round action to do so) then you can accurately hit any part you can reach. If this is sufficient to kill the creature (either directly through hit point damage or by the failed Fortitude save) you’ve landed the killing blow you wanted.
You take a penalty to hit (probably -4 or -5) you can make a called shot. If you hit and the damage is sufficient to kill the creature, you made the shot you wanted. If you hit and the damage is not enough to kill the creature, it’s a ‘normal hit’ — you may or may not have hit the part you wanted, but since it wasn’t enough to kill, it doesn’t matter.
The above rules don’t particularly work for targeting other specific body parts (such as aiming for the eyes to blind the victim, or trying to amputate a hand in a fight), but could probably be extended. Perhaps a coup de grace or successful called shot that results in a critical? Coup de grace is an automatic critical, so the behavior is consistent between the two.
Examples
In all cases you must either get the victim while helpless and coup de grace, or make a called shot as described above.
Staking a Vampire
Undead are normally immune to coup de grace (immune to criticals). For this purpose I would rule that the vampire doesn’t take any additional damage from the coup de grace or be required to make the Fortitude save, but there would be no penalty for the called shot. If the called shot is successful and you manage to ‘kill’ the vampire, it’s staked.
Decapitation
If the coup de grace or called shot is sufficient to kill the target, he may be decapitated. In Dragaera this prevents revivification, and may prevent raise dead from working in an RSRD (raise dead requires a more or less intact body). In both cases dismemberment after death would work too.
Bloodtheft
In the Birthright setting being killed by being pierced in the heart with a tighmaevril weapon results in the killer being able to steal the victim’s bloodline strength. A fatal coup de grace or called shot to the heart with a tighmaevril weapon would allow bloodtheft.