What happens if you attempt a coup de grace with a weapon that deals nonlethal damage, such as a sap or a weapon with the merciful property? Is the coup de grace still automatically a critical hit? Is the target required to make a Fortitude save? If so, what’s the DC, and what happens if the target fails? What happens if you use a normally lethal weapon to deal nonlethal damage as a coup de grace?
This question takes us beyond the rules. You could rule that you cannot deliver a coup de gace with nonlethal damage, but if you want rules for using nonlethal damage in such an attack, try these:
When you attempt a coup de grace with a weapon that deals nonlethal damage, you automatically hit and inflict a critical hit. Note that you cannot deliver a coup de grace to a creature that is immune to critical hits. Calculate the nonlethal damage from the resulting critical hit just as you would normally. If the nonlethal damage isn’t sufficient to render the subject unconscious (see page 153 in the Player’s Handbook), the subject should make a Fortitude save (DC of 10 + the nonlethal damage dealt). If the save fails, the subject is rendered unconscious. The subject immediately suffers enough nonlethal damage to make his current nonlethal damage total equal to his current hit points +10. For example, you perform a nonlethal coup de grace on a helpless gnoll that currently has 12 hit points. You hit the gnoll and deal 10 points of nonlethal damage, not enough to knock out the gnoll. The gnoll, however, must make a DC 20 Fortitude save. If the gnoll fails the save, its nonlethal damage total immediately rises to 22 (current hit points +10), and it falls unconscious. This is roughly the equivalent of being killed when you fail your saving throw against a lethal coup de grace, since death occurs at –10 hit points.