Hello, I am not a native English speaker, so I would first like to apologize if I can't express the proper tone.

As the title says, below is the Feature; it is a very rough idea and I would like your opinion about it, and as a DM would you allow for something like this to exist in your game. Or if something like this already exists would kindly please point me to it. Thank you.

Name: Iaido

Pre-requisite: Proficiency with the Equipped Single-Handed Martial Melee Weapon.

Once per long rest "you" can enter into "Drawing Stance" skipping your turns until a hostile "Creature" enters into your melee attack range triggering an Attack of Opportunity, ignoring Dashing/Disengaged and/or any Feature the prevents an AoO reaction.

Rolling a 16 or Higher on the Attack Roll (w/o Modifiers) turns the Attack into a Critical Hit.

The hostile "Creature" will make a Dexterity Saving Throw(w/o Modifiers); If successful the "Creature" takes no Damage. The "Creature" can roll for advantage on the Saving Throw if applicable.

It seems overly complicated and has a lot of unnecessary downsides. In practice it seems very similar to the Sentinel feat... Essentially you're foregoing your action in order to set up an attack that crits much more easily, which is useful, but giving the target a chance to dodge 100% of the damage makes it kind of useless. As well, making this once-per-long-rest really would make it terrible, since it is only useful in very specific conditions.

I think I'd change it to "As an action, you enter a defensive stance. While in this stance, enemies provoke an Attack of Opportunity if they enter your melee range for the first time, even if they took the disengage action. This Attack of Opportunity is considered a critical hit if you roll 16 or higher"

The only cost necessary for a feat like this is that it consumes your action for the round. It's essentially like holding your attack, but giving a decent bonus to it.

I've been thinking about a version of the 3.5 edition iajitsu focus for a while which you are welcome to use or amend:,

Feat Name: Iajitsu Focus (I believe this would translate to English as Seasoned Focus)

Requires the ability to use Ki points and sword that deals slashing damage.

Description: You are the master of drawing a weapon and striking a weapon blow in one swift and elegant movement. This is most commonly seen in one on one duels of honor where it is considered the height of skill to only need to strike once.

Effect: You must have a sword sheathed (in a scabbard or similar holster) to be able to perform this attack. You can use your action to  declare an Iajitsu strike, doing so allows you to move upto 5ft towards a creature you can see, draw your sword and make one attack. the 5ft of movement is deducted from your normal speed allowance. The attack scores a critical hit on a natural 19-20 and if the attack hits you can expend Ki points to deal additional damage, for each Ki Point expended you add 1d6 damage to the damage roll. If the attack is a critical hit, these additional damage dice are also doubled.

DM note: for purposes of this skill I always think of a samurai using a katana and would allow a monk using a katana and treat it as a long sword.

Thank you for the input, it's been very Educational.

The colossal crit range on the idea has me Fry Meme-ing a bit, but overall I think Transmorpher's idea is sound.  Using your action to allow you a new way of using your reaction, and striking only once because of it, is normally super inefficient.  If one is trying for the whole Iaido single-stroke battle though, I could see this being a thing. 

Whether or not I'd allow a five-point crit range depends on whether I'm using base crit rules or not, if any of the various empowered crit rules people use are in play I'd be leery of it.  Not to mention the fact that as a feat, this'd be fair game for rogues or palladalladingdongs digging for an abusable Super Mega Ultra crit unless you put class, background, or training restrictions on it.  Fighters that can trade extra attacks for a single attack with broad crit range aren't likely to break anything.  Rogues that don't trade anything for it, or palladalladingdongs that trade one attack for a quintupled chance at Crit Smite Bullfudgery, are an entirely different issue you'll want to think about before making this official at your table.

I was thinking of making this an exclusive perk under the Battlemaster/Samurai Archetype. Thank you, will think deeper on this.

Iaido/Iaijutsu (from my understanding) is a reaction based technique, where you wait to strike until your opponent shows hostile intent (attacks). So Sentinel / Polearm Master should be decent starting points. Both of these let you make Attack of Opportunities outside of normal conditions. I like that you have it limited to one-handed martial weapons since that will prevent Polearm Master abuse.

 First Write-Up

 I don't think this gives enough to be worth a feat though. It needs something else, I think increased crit is too powerful. To further emphasise the reactionary nature, how about if you don't attack, you can get advantage?

this give us:

Iaido

Prerequisite: Proficiency with a one-handed martial melee weapon.

• If you don't make any attacks on your turn, you have advantage on any opportunity attacks you make until the start of your next turn.
• When a creature within 5 feet of you makes an attack against you, you may use your reaction to make a melee weapon attack against the attacking creature. Your attack must be with a one-handed martial weapon and you may draw or sheathe it as part of this attack.

Edit: adding it to reactions I think would be too strong

I do think that balances it pretty well... it doesn't guarantee that the attack eats up your action (since you can still do things other than attacking), and granting yourself advantage seems like it will be more balanced than increasing crit chances.

Counterpoint: Eldritch Knights, or other characters capable of casting spells.  They can cast freely (so long as it's not a spell attack roll), and then reap the benefits of this version of Iaido on their turn.  Arcane Tricksters could cast their spells, then gain a Sneak Attack-granting advantage attack as a free reaction.

Yeah, I thought about what the wording should be. Does it need limited like invisibility and "no hostile actions."

My concern was that I didn't want it to become too wordy and also to not hamper characters too much.