I feel like the rules of 5e (I'm not familiar with other editions) are really lacking in department of stealth opportunities.
I'm mostly talking about one shot kills, but not necessarily. Examples:

• A PC manages to sneak behind a guard. Cinematically (as I understand it doesn't really work like that in real life) he would hit him on the head with a stone to make him unconscious, so the party can all walk over him to the next "checkpoint".


• PCs sneak up on a sleeping cult leader, and would like to put a dagger through his throat.

I really don't want to ban such actions from my games, they are cool, but rules don't really support them also. Sneaking up to an NPC takes at most a couple of Stealth checks, so in terms of gameplay it's really easy to do. I can't think of a good check to give players for putting a dagger in a sleeping human. It's really a trivial action. Hitting someone with a stone is a bit harder, but not that much, so the same concerns apply.

I don't think using "Attack" rules works - firstly, rolling attacks implies that you can either miss, or not make any damage through the armor, which is really not the case with a sleeping/unaware human. Secondly, if you roll attack, then you probably should roll for damage too, and even something like 20 damage (40 on a crit), which is really a lot for a single physical attack (something like 12 from rolling, 4 from STR, 4 from magic enhancements), is only enough to take out of action weaker characters like Thug (CR1/2, 32 HP). A Bandit Captain (CR2, 65 HP) will withstand such an attack somewhat easily. And, more realistically, you'll get something like 5 damage (stones and stakes are improvised weapons - 1d4, no proficiency, no magical damage on top), and that's barely enough to kill a commoner.

So, do you have suggestions on any good mechanics, that would make it possible? They have to both feel fair to the players (no misses/low damage on unmoving unprotected objects), and not make it a walk in the park (one stealth roll killing a Gladiator (CR5, 112 HP).

Suggestions for stealthy incapacitation

I read three parts to your question:

1. How can I make sure sneaking up to strike an enemy isn't too easy?


2. How can I allow a single-blow kill?


3. How can I allow a single-blow knockout?

Sneaking past a guard is significantly easier to than sneaking close enough to him to hit him on the head with a rock or slash his throat. Page 174 of the Player's Handbook and page 238 of the Dungeon Master's Guide both list descriptions of difficulty classes, though neither provides a list of examples as to what a "hard" versus "very hard" difficulty class for a skill check might look like. In order to achieve the right balance in your game you can 1) establish a difficulty class that is not so easy to achieve and 2) make sure the players see a serious downside to risking trying to sneak up for a quick kill that might make them consider other options. On the flip side, remember that you are deciding when a check is even needed, and if there is no chance of failure, allow automatic success, as mentioned on page 238 of the DMG.

While earlier editions of the game allowed for a coup de grace, whereby a helpless creature could automatically be dispatched with a melee weapon, 5e specifically does not. The condition unconscious allows advantage on attack rolls and automatic critical damage if attacked from within 5', but not automatic death. If you want it to work differently you will need to house-rule it, or borrow from another edition of the game. Page 133 of the 3rd edition Player's Handbook has rules for executing a coup de grace, you might make as a house rule.

It's important to note that an unaware creature is different from a helpless creature. A coup de grace could be performed on an unconscious or sleeping target, not on a creature that is taken unawares, but nevertheless able to react.

The 5e rules provide for a creature to be knocked out only when reduced to 0 hit points and the attacker decides to knock it out instead of killing it. For this to work as a single blow against creatures with more than a handful of hit points, you'll need to adopt a house rule.