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If a character or creature has an AC that with all bonuses requires a 20 to hit does that mean that every hit is a crit? Or alternately the creatures AC is so high that with all bonuses no hit would be possible except that a 20 always hits. So if a nat 20 is rolled you hit and crit and then the only way you can hit the impossible to hit creature also results in a crit.

I feel like the impossible to hit would grant you a hit on a nat 20 but not the crit. What is correct?

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    \$\begingroup\$ Hi Cory! Welcome to our stack! Please take the tour to learn more about how we operate and you can visit the help center for more information,. \$\endgroup\$ – NautArch Nov 19 '20 at 16:52
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    \$\begingroup\$ "I feel like the impossible to hit would grant you a hit on a nat 20 but not the crit. What is correct?" Can you elaborate on where this feeling comes from? \$\endgroup\$ – Cubic Nov 19 '20 at 18:06
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    \$\begingroup\$ Related: Does a critical hit from an expanded crit range always hit regardless of AC? \$\endgroup\$ – Ifusaso Nov 19 '20 at 18:08
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    \$\begingroup\$ If a creature's AC makes it impossible for you to hit it with all mods and bonuses but a natural twenty always hits I feel like the natural twenty in this case only grants you the hit not the crit since you are only able to hit it because a nat twenty always hits. Example: a first level fighter with a 16 strength using a long sword comes upon a creature with a 25 AC (not likely but bear with me) the only way for the fighter to score a hit is a twenty +3 for strength +2 for Proficiency = 25. so its a crit every hit seems odd \$\endgroup\$ – Cory Jenkins Nov 19 '20 at 18:18
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    \$\begingroup\$ @CaptianObvious What's curious about that? Different games (can and do) have different rules. \$\endgroup\$ – TylerH Nov 20 '20 at 14:04
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You've written:

So if a nat 20 is rolled you hit and crit and then the only way you can hit the impossible to hit creature also results in a crit.

That's right -- you've understood the rules correctly.

You've also written:

I feel like the impossible to hit would grant you a hit on a nat 20 but not the crit.

That might be a good house rule, but it's not a part of the official rules.

If you run a game, you could make that be a house rule. You probably shouldn't, though, because:

  • every house rule you add is one more thing for your players to keep track of, and you'd probably rather have them think about the plot of your game
  • it's very unlikely that your players will ever fight a monster that requires a natural 20 to hit, so this shouldn't come up in practice
  • if you do run a battle where one side requires a natural 20 to hit, it will be an extremely imbalanced and one-sided battle already, and you won't want to have a houserule that makes it even more one-sided.

Very few monsters have an AC that is that high. The highest I could find was the Tarrasque (CR 30) with an AC of 25; the second highest was the Warforged Colossus (CR 25) with an AC of 23. Most fourth-level characters will have +6 to hit, so unless you're running a story where third-level characters fight the Tarrasque, this situation should never come up.

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A natural 20 on an attack roll disregards armor class entirely.

From the rules for attack rolls (emphasis mine):

If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. This is called a critical hit, which is explained later in this section.

A critical hit totally disregards armor class. Since this hit is called a critical hit, we consult the rules for critical hits:

When you score a critical hit, you get to roll extra dice for the attack's damage against the target. Roll all of the attack's damage dice twice and add them together. Then add any relevant modifiers as normal. To speed up play, you can roll all the damage dice at once.

The DM is welcome to houserule this.

The intro the the Dungeon Master's Guide says:

The D&D rules help you and the other players have a good time, but the rules aren’t in charge. You’re the DM, and you are in charge of the game.

If you are the DM and you don't like this particular rule, you have the liberty to change it. But as with all houserules, discuss it with your players first.

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    \$\begingroup\$ Then my issue is more, I don't think the bonus damage is warranted or earned when the only way to hit is with the natural roll. It seems like there should be some additional benefit to the higher AC. A massively armored fighter wades into a sea of kobolds so if any manage to hit its a crit? \$\endgroup\$ – Cory Jenkins Nov 19 '20 at 18:27
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    \$\begingroup\$ Yes, that's what the rules say, as quoted in my answer. \$\endgroup\$ – Thomas Markov Nov 19 '20 at 18:28
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    \$\begingroup\$ @CoryJenkins Think about it the other way around: the only hits that will have an effect are the big wallops/critical hits \$\endgroup\$ – G. Moylan Nov 19 '20 at 21:56
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    \$\begingroup\$ If you think of this in real-world terms, a knight in plate armor taking on a peasant in none, the peasant doesn't stand a chance, until he hits a joint, or through the visor... in that case, the arm/leg comes off/is useless, or the fight is absolutely over. \$\endgroup\$ – CGCampbell Nov 20 '20 at 13:10
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I was going to make this a comment, but it became more involved...

What you are calling "impossible" is merely "difficult for the situation"

In the comments you gave an example:

If a creature's AC makes it impossible for you to hit it with all mods and bonuses but a natural twenty always hits I feel like the natural twenty in this case only grants you the hit not the crit since you are only able to hit it because a nat twenty always hits. Example: a first level fighter with a 16 strength using a long sword comes upon a creature with a 25 AC (not likely but bear with me) the only way for the fighter to score a hit is a twenty +3 for strength +2 for Proficiency = 25. so its a crit every hit seems odd

But that's not an "impossible" AC to hit. It is only very hard.

If that fighter also had a Bless spell on them, it would give an additional d4 meaning they could potentially hit on a 16! So long as they also rolled a 4 on the Bless die roll. Or someone gave them a enchanted (+1, +2, or +3) weapon. Or they were using a bow, had a 16 Dex, and had the "Archery" fighting style for a +2 bonus.

It just so happens that for a given scenario, to hit a certain AC, it requires a 20 plus all of a players' current bonuses/mods is just math. The crit is still a crit.

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