Driving home, I was thinking about the somewhat contrived situation where you have achieved total surprise on a high-level character – someone or something with lots of HP.
You can certainly roleplay a coup de grace, or a sneak attack, and your GM can say “Yep, you know he dead.”
But I’m pretty sure that 5e doesn’t formally work that way. Sure, it’s an automatic critical, but by and large, if you can’t do huge damage, you’re in for a fight.
That’s not new for D&D.
I was wondering though, if there might be a way to deal with some of that, in combat or not.
Here’s the thought. Instead of rolling double dice of damage on a critical hit, instead you may choose to have the foe make a CON saving throw.
If he fails, he gets an X mark in his death-check tally. Fail two, and he’s unconscious and/or reduced to 0 HP. The third and last kills, as normal. A crit fail counts as two marks, a crit success removes a mark if one already exists (“I got better.”)
What DC for the CON save? Probably something like 8+the attacker’s proficiency bonus. Maybe even lower given some of the issues below. If you wanted it to be more attack-based, the DC could be either 4 plus the attack’s hit bonus (so skilled combatants are harder to avoid the crit factor), or even a contest of weapon skill vs. CON.
This very well might be overpowered. But while certainly it probably needs much playtesting, let’s throw down a few surface level pros/cons/observations
- Two critical hits and two failed saves to any foe and he’s KO’d. This might need more conditions, like “you must be advantaged and score a crit,” though of course the odds of a crit are double with advantage, so that might not be a net restriction.
- This would allow, potentially, for the classic ambush scenario to be resolved successfully with a few hits incapacitating foes.
- Crit fishing would be a thing, though given a roughly 50% save rate, you’ll need four of them in one fight to drop a foe.
- There probably needs to be a way to recover some of those with healing, second wind, or some sort of defensive action (disengage followed by some sort of recover action, that you can only take if you’re not being menaced in combat?). So that way you can actually shrug off a crit.
- Might make CON even more important than it already is. I don’t know if this is a good or bad thing.
- Crits become quite powerful, and the Champion archetype, with his “crit on 19-20” and then even 18-20 at high level becomes a perhaps ridiculously effective hitter here – with a 28% chance of a crit on each blow and something like DC14 on the saving throw, you’ll wind up scoring a death-track critical one in about every seven or eight blows thrown. And at 2-3 attacks per round (four at 20th level), he’s dropping a foe of arbitrarily high HP every two or three rounds. 
- It’s a bunch of rolling in what is supposed to be a rules light-ish game. It’s not supported by Roll20 or any of the VTTs out there, and it introduces a measure of analysis paralysis (should I just take double damage, or go for a KO?)
- You must have had advantage on your attack (good position)
- You must roll a critical hit (good luck; that’s a 10-20% chance if you also have advantage)
- You muse roll more damage than your foe’s base HD plus their con bonus. So a barbarian with CON 18 (+4) would need to be hit with an attack that did more than 16 HP in order to trigger a potential CON save
- Your foe must make some sort of saving throw – and I’m tempted to say CON save OR applicable weapon/defensive skill (like a Parry), whichever is better.