Ruin Explorer
Legend
I'd be interested to see what mechanics you come up with.I absolutely love this idea nested in your post for situations like knife at victim's throat or 10 crossbows aimed at PC which tension in scenes like these are undone by the hit point mechanic.
I'm definitely going to incorporate something like this at my table going forward.
WWN does the follow for stealth assassination-type stuff (I think it's okay to reproduce this, it's in the free version of WWN):
Execution Attacks
A target that is completely unaware of danger is vulnerable to a quick and bloody death, no matter how great their martial prowess or how thick their armor. An Execution Attack gives an assailant an opportunity to slay a foe with a single well-placed arrow or blade. Setting up such an attack requires a full minute of preparation. Archers, gunmen, and other ranged attackers must spend it judging distance, wind, and details of aim, while melee assassins must use it to drift up to the target and position themselves in the exact right place for the attack. Melee assassins must use a weapon for an Execution Attack, unless they have such special training as to make their unarmed attacks unusually lethal. If the target is spooked, the opportunity is lost. Once the preparation is complete, the assailant may use a Main Action to attack. The target’s Armor Class is irrelevant, assuming the attacker is using a weapon that can hurt the target. A melee Execution Attack will always hit. A ranged Execution Attack requires a Dex/Shoot skill check against a difficulty of 6 for a point-blank shot,8 for one at the weapon’s normal range, and 10 for a shot at extreme range. A Warrior can use their Veteran’s Luck ability with this skill check, but it only allows a reroll on a failed check rather than forcing an automatic hit. If the Execution Attack hits, the target must make an immediate Physical saving throw at a penalty equal to the attacker’s combat skill level. If they fail, they are Mortally Wounded on the spot, or knocked unconscious if the attacker was using a plausibly non-lethal weapon. If they succeed, the weapon still does its maximum damage. Attacking an unconscious or incapacitated target will always leave them Mortally Wounded.
TLDR - If you hang out for 1 minute getting ready to kill your target (unseen, undetected), you can make an attack that potentially one-shots them. There are some other mechanics that interact with this too (to make it easier or impossible against certain targets). I would personally consider reducing the 1 minute circumstantially and/or on a class-based basis.