The 9th Level Enchantment spell Power Word Kill outright kills any creature with less than 100 hitpoints.
I've had a look, but I can't see anything that would be similar via any melee action of feat, creature-, or PC ability.
So my question is: Is there anything that allows a melee attack to strike down a foe to either 0 hit points (rolling death saving throws) or outright kill them using one attack?
Effects with saving throws would also answer the question.
Best Answer
There's a class of legendary magic weapons, Vorpal Swords:
It has a bunch of limitations, but against most creatures it has 1 in 20 chance of instantly killing your opponent.