Yeah, unfortunately the issue with a garrote is that insta-kill weapons/techniques don't really mesh well with the 5e combat system on the player side. A lot of DM's likewise don't allow "I come from behind and cut his throat" type moves with out of combat sneaking Rogues as well, to give another example. There are, by design, only a handful of specific ways to bypass the standard "reduce their HP to 0" method of killing an enemy, and they're generally less practical. People are of course free to homebrew their own weapons/techniques/etc, but it's a fine balance. Make it too easy, and combats can be trivialized or at least the people who use the technique will be very dominant in combat. Make it too difficult, and it could either end up never being used or the player trying to use it could become upset that their cool thing rarely works.

How about:

Special weapon (finesse)

Make an attack from behind

If the attack is successful, they both become restrained and the victim is unable to speak or yell

At the beginning of each of their turns, the target takes 1d6+str/dex dmg, as long as they are still restrained. 

They can break free by making a successful opposed roll at the beginning of their turn when the damage has been dealt. (Acrobatics or Athletics vs attacker's attack)

(Note that the first round is sort of wasted in the grappling and they can break out of it)

(If there is no dmg die, then a normal punch in the face deals the same damage as a weapon designed for murdering people)

(I upped the DMG die to d6, because it has limitations and also keeps the attacker restrained, and they cannot use multiattack.)

(Now because I really like this weapon concept, I would probably give the rogue a homebrew feature or make a feat that allows them to add their rogue bonus to the first damage roll. But generally the weapon can be used by a barbarian too)

(If the damage feels too big: You compared it to a commoner's HP. I wouldn't worry about the commoner's HP. If you plan to use the weapon at all, make it somewhat scale throughout the game and be useful in combat in general. The commoner will die instantly either way.)

(In this the damage represent both physical damage and suffocation. If they have 15hp, this will still work, just more slowly)

This is not the simplest way, but to me it's simple enough and it's a very situational weapon to use, so I would probably either not make it at all or make it actually worthwhile to use. :)

Restrained is still a pretty strong condition to be able to attempt to inflict at will. The Grappler feat takes two turns to inflict it. Also, "behind" is a word that doesn't really have meaning in baseline 5e combat. I know there's some optional facing rules in the DMG, but I've never heard of anyone actually using them in a game.

One factor I see missing is Size.

Garrotes require that an attacker enter a target's space. 

... you can move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you. Remember that another creature's space is difficult terrain for you.

This kinda puts a bit of a damper on the use of garrotes as a whole unless you use the grapple function for at the least the attack.

Grapple functions in this case would also be presumed to involve a "strangling" attack (like a headlock) so the general rules should be consistent with other form of similar attack -- solve this, and you can add "strangle" to unarmed combat options, essentially.

So I would do an initial attack of Grapple. If successful, you have a grip on the opponent, and you can begin to strangle them (choking/suffocation) -- so that would essentially start on the next round.

And as I scan up I come to realize this is one of the threads I normally avoid like the plague, because I hate posting to threads from over six months old.

Dang nabbit.

I would go with the surprise attack. Hand to hand. Then go through the chocking rules. The victim getting the chance do anything at disadvantage until they fall unconscious. At that point the attacker can either keep the garrote on them and working to keep the victim from passing any death save.

 A few rounds is a small price to pay for killing a high level character.
But then again most high level characters have friends around to help them out.

And neck armor is a real thing in real life. Just add it to the game.

Also local authorities would not like to find a garrote on any character since its only use is as a n assassins weapon.

they work well. Thats why they are used. But not in a combat situation.

Reading through this, agree that just 2 rounds of trying to escape is too little, and 51 is too many!

Here's my suggestion:

Garrote (tool). As an action, you can use the Garrote to attempt to choke a creature which is not more than one size larger than you, which is currently grappled by you. When you do so, the creature you have grappled immediately starts choking (see Suffocating), and cannot speak. Eact turn thereafter, you must use your action to apply the Garrote, or the creature will be able to breathe again. If your grapple ends, the creature can breathe again. If a creature would drop to 0 hitpoints as a result of the Garrote, it instead falls unconscious for a number of hours equal to 1d4 plus its constitution modifier. This condition ends if it takes damage or recovers any number of hit points, provided that it can breathe. If you continue to use your actions to apply the Garrote for 3 further rounds, it will instead drop to 0hp.

Unless I'm missing something, this adds one turn to the whole system - you need to first Grapple the creature, using an action, then use each of your subsequent actions to choke it. Meanwhile, it can be attempting to escape the grapple, or dragging you around to attack your friends, or just attacking you! So, a creature with a con mod of +1 or less gets 1 round to escape the grapple, 1 round to escape after garrotting, and then drop. Assuming you're fighting a single foe with low constitution, that's not crazily unreasonable.

Also, if you don't want to kill them, you can instead leave them unconscious. This makes it less dangerous as a combat tool, as it means a minimum of 5 rounds to actually kill something. If you knock it out, then someone stabs it, it wil lregain consciousness, so it's not just an insta-win for the party either!

With a garrote I would also allow a smaller creature to garrote a larger one. Maybe one size larger.

Just to add to my one, I would also consider adding a feat to work with it to allow you to start garotting on the same turn as the initial grapple, not sure if it'd need more than that to be worth it!

It occurs to me that another thing missing is the "ranged strangle" attack.

A garrote could also be a whip, or a kind of weighted fabric used as a strangling cloth.

This could engage the "moving" rules during a grapple, as the two folks struggle to keep/free the wrapped neck.

I would not give it range.hey are a thin wire with two handles. You could use a narr*ow cloth or rope but a wire is better.

They are typically wrapped around the neck from the back and crossed over at the handles so they can not be loosened or a finger can not get under to leave room for blood flow.
All the attacker needs do after that is hold on and hold it tight.

Quote from ThorukDuckSlayer >>

Reading through this, agree that just 2 rounds of trying to escape is too little, and 51 is too many!

Here's my suggestion:

Garrote (tool). As an action, you can use the Garrote to attempt to choke a creature which is not more than one size larger than you, which is currently grappled by you. When you do so, the creature you have grappled immediately starts choking (see Suffocating), and cannot speak. Eact turn thereafter, you must use your action to apply the Garrote, or the creature will be able to breathe again. If your grapple ends, the creature can breathe again. If a creature would drop to 0 hitpoints as a result of the Garrote, it instead falls unconscious for a number of hours equal to 1d4 plus its constitution modifier. This condition ends if it takes damage or recovers any number of hit points, provided that it can breathe. If you continue to use your actions to apply the Garrote for 3 further rounds, it will instead drop to 0hp.

 

Unless I'm missing something, this adds one turn to the whole system - you need to first Grapple the creature, using an action, then use each of your subsequent actions to choke it. Meanwhile, it can be attempting to escape the grapple, or dragging you around to attack your friends, or just attacking you! So, a creature with a con mod of +1 or less gets 1 round to escape the grapple, 1 round to escape after garrotting, and then drop. Assuming you're fighting a single foe with low constitution, that's not crazily unreasonable.

Also, if you don't want to kill them, you can instead leave them unconscious. This makes it less dangerous as a combat tool, as it means a minimum of 5 rounds to actually kill something. If you knock it out, then someone stabs it, it wil lregain consciousness, so it's not just an insta-win for the party either!

 
I just apply the garrote on the initial grapple, and as long as the grapple is maintained the choking happens automatically every turn. It works rather well.

Quote from TheGnome5786 >>

I would not give it range.hey are a thin wire with two handles. You could use a narr*ow cloth or rope but a wire is better.

They are typically wrapped around the neck from the back and crossed over at the handles so they can not be loosened or a finger can not get under to leave room for blood flow.
All the attacker needs do after that is hold on and hold it tight.

Yes, that's the traditional western depiction of a garrote.

I was looking at the larger world and considering the mechanics of how strangulation operates as derived from the use of a garrote.

There are more ways to strangle someone with a weapon than just using a garrote, and ore forms of garrote out there.