Swarmkeeper
Hero
What @billd91 said
Also, this passage from the DMG (pg 236) (emphasis mine):
So, no House Rule needed to adjudicate an automatic success for a PC insta-kill of an enemy in a situation where the DM deems it appropriate, @Maxperson. Not that there is anything wrong with House Rules. I've got several House Rules. But this is not one of them.
Also, this passage from the DMG (pg 236) (emphasis mine):
Remember that dice don't run your game-you do. Dice are like rules. They're tools to help keep the action moving. At any time, you can decide that a player's action is automatically successful. You can also grant the player advantage on any ability check, reducing the chance of a bad die roll foiling the character's plans. By the same token, a bad plan or unfortunate circumstances can transform the easiest task into an impossibility, or at least impose disadvantage.
So, no House Rule needed to adjudicate an automatic success for a PC insta-kill of an enemy in a situation where the DM deems it appropriate, @Maxperson. Not that there is anything wrong with House Rules. I've got several House Rules. But this is not one of them.