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 or feat. Creature of 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.