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(Inspired by this question about adamantine armor)

The item description for adamantine armor states:

While you're wearing it, any critical hit against you becomes a normal hit.

The item description for a vorpal sword states:

When you attack a creature that has at least one head with this weapon and roll a 20 on the attack roll, you cut off one of the creature's heads. The creature dies if it can't survive without the lost head.

A roll of a natural 20 is also a critical hit, which adamantine armor reduces into a regular hit. So, if a creature wearing adamantine armor is hit by such an attack from a vorpal sword, does the adamantine armor also prevent the beheading property of that sword?

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    \$\begingroup\$ Are you attacking a manxome foe? Did it come whiffling through the tulgey wood? \$\endgroup\$ Commented Oct 20, 2018 at 15:42
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    \$\begingroup\$ Oh, geez, this question gives me flashbacks to 3.0, where Vorpal activated on a crit and there were all these keen-vorpal-scimitar-prestige class builds to try to expand the crit range until you could lop heads on a 15+. Ever since 3.5 it's been "on a natural 20" instead! \$\endgroup\$ Commented Oct 22, 2018 at 1:46
  • \$\begingroup\$ Maybe of interest: Origin of the Vorpal Sword - even the earliest versions used explicit language of numbers that decapitate \$\endgroup\$ Commented Feb 11 at 4:05

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No, the creature still loses its head

The effect of the vorpal sword is explicit:

...and roll a 20 on the attack roll, you cut off one of the creature's heads.

This wording in the item description implies that the beheading effect is completely distinct from the effect of a critical hit. Reducing the result of the attack from a critical hit to a regular hit doesn't change the fact that the attack rolled a natural 20. So, adamantine armor won't stop the sword from beheading you.

Other weapons with similar wording behave the same

Items like the sword of sharpness and mace of smiting have similar wording. As such, they work identically. If you roll a natural 20 on your attack roll, then you apply the extra effects from the item, regardless of whether or not the result is a critical hit.

However, effects that apply "when you score a critical hit" are negated

For example: The nine lives stealer description states:

If you score a critical hit against a creature that has fewer than 100 hit points, it must succeed on a DC 15 Constitution saving throw...

In this case, when the attack scores a critical hit against you, the critical hit becomes a mundane hit. So, the extra effect from this kind of weapon doesn't apply since the attack didn't score a critical hit.

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    \$\begingroup\$ I think the Vorpal has such a wording to avoid balance problems with Champions, and the Adamantine armor is meant to protect against it, but only a tweet could prove me right. \$\endgroup\$
    – András
    Commented Oct 20, 2018 at 14:23
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    \$\begingroup\$ Crawford confirms this interpretation about the vorpal sword's "natural 20" requirement here and here. \$\endgroup\$
    – V2Blast
    Commented Oct 20, 2018 at 17:26
  • \$\begingroup\$ @V2Blast I am talking about a confirmation of Adamantine. I think Adamantine is meant to protect against Vorpals, but Champions are more common, so they chose the lesser evil. \$\endgroup\$
    – András
    Commented Oct 20, 2018 at 19:46
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    \$\begingroup\$ @András, a Vorpal Sword is a legendary item, while adamantine armour is an uncommon item, so I would assume the opposite: the vorpal sword would still work, by the disparity in power level. If they had intended the vorpal sword to be blocked by adamantine armour, they probably would have written a clause saying something along the lines of 'creatures immune to critical hits are unaffected by this effect'. The negation of critical hits in general, not counting the niche case of vorpal swords, is sufficient for an uncommon item. But this is now delving into opinion. \$\endgroup\$
    – BBeast
    Commented Oct 20, 2018 at 20:48
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    \$\begingroup\$ @András: I was "confirming" the answer, not your comment. I don't think adamantine armor is meant to protect against vorpal weapons. \$\endgroup\$
    – V2Blast
    Commented Oct 20, 2018 at 21:40
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A critical hit and a 20 on the attack roll are two different things.

The sword's description states (emphasis mine):

When you attack a creature that has at least one head with this weapon and roll a 20 on the attack roll, you cut off one of the creature's head.

Beware: it does not say a 20 on the d20, but a 20 on the attack roll. Per PHB, an attack roll is a d20 roll plus modifiers (bold & italic mine):

Ability checks, attack rolls, and saving throws are the three main kinds of d20 rolls, forming the core of the rules of the game. All three follow these simple steps.

1. Roll the die and add a modifier. Roll a d20 and add the relevant modifier. This is typically the modifier derived from one of the six ability scores, and it sometimes includes a proficiency bonus to reflect a character’s particular skill. [...]

2. Apply circumstantial bonuses and penalties. A class feature, a spell, a particular circumstance, or some other effect might give a bonus or penalty to the check.

[...]

This is confirmed also on the Attack Rolls section in the PHB:

To make an attack roll, roll a d20 and add the appropriate modifiers.

Per the rules as written, if a character with a vorpal sword has a bonus to hit equal to +8, then rolling a 12 on the d20 provides an attack roll of 20, triggering hence the sword's power. But it is not a critical hit.

There are some magic items (weapons) that explicitly require a critical hit and not a 20 on the attack roll, for example the nine lives stealer.

Adamantine Armor does not protect from vorpal sword's effect.

As stated in the previous posted answer, Adamantine Armor protects from critical hits, which are not suitable triggers of the vorpal weapon: the target of the attack still loses theirs head in case of a 20 on the attack roll.

How does it affect the probability to trigger the sword's effect?

Consider the example above for a generic character: with a bonus of +8 a 12 on the d20 gives an attack roll of 20. The probability to get this outcome is 1/20. The probability of getting a critical hit is 1/20. The probabilities are the same, thus adopting the "critical rule" for the vorpal sword does not affect the probability.

There are differences when some bonuses can be applied to the attack roll, for example the 1d4 from a Bless spell (see point 2 in the definition of d2 rolls). In this case, under the same +8 base bonus, the probability to have a 20 on the attack roll is1 1/10.

Moreover, there are some subclasses (Champion, Oath of Devotion Paladin) that benefit from improved range for critical hits: the DM has to decide how to treat this subclasses under using crit hits for triggering the head-severing effect (indeed, it doubles the probability for these subclasses).


1 This is given by $$ \begin{eqnarray*} P(20|\text{Bless}) &=& P(8 \text{ on d}20)\cdot P(4 \text{ on d}4) +\\ &&P(9 \text{ on d}20)\cdot P(3 \text{ on d}4) +\\ &&P(10 \text{ on d}20)\cdot P(2 \text{ on d}4) +\\ &&P(11 \text{ on d}20)\cdot P(1 \text{ on d}4) +\\ &&P(12 \text{ on d}20)\\ &=& \left(\frac{1}{20}\cdot\frac14\right)4 + \frac{1}{20}\\ &=& \frac{1}{20} + \frac{1}{20} = \frac{1}{10}\\ \end{eqnarray*} $$

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  • \$\begingroup\$ You are correct that 'the attack roll' is the total of the d20 plus mods - this is confirmed, for example, by the text of cutting words. However, the sword does not say it decapitates when you 'have a 20' / 'make a 20' / 'get a 20' on the attack roll. Instead, it says you decapitate when you 'roll a 20 on the attack roll'. This suggests to me that it is using the first roll as a verb, and the second roll as a noun, and they do not mean the same thing. Rather, I think it means 'When you roll a 20 on the d20 component of the attack roll, regardless of the final attack roll total.' \$\endgroup\$
    – Kirt
    Commented Feb 10 at 17:28
  • \$\begingroup\$ @Kirt if you look at the rules, attack roll, saving throws and ability checks are "d20 rolls", which are not simply a roll of a d20, but they are defined as "d20 roll plus mods". So the roll of an attack roll is the roll of a d20 plus mods. For the Bless spell, I am going to add a bit. \$\endgroup\$
    – Eddymage
    Commented Feb 10 at 18:17
  • \$\begingroup\$ I agree that a "d20 roll" (sustantivivo) is the result of the roll of the d20 plus the mods. I just think that the description of the sword "roll a 20 on the attack roll" uses the first, bolded roll to mean a verb, specifically the act of rolling the d20 and the result of that, before the mods. You use it similarly when you say "rolling a 12 on the d20 provides an attack roll of 20". This is my interpretation, of course, and we can disagree there. \$\endgroup\$
    – Kirt
    Commented Feb 10 at 19:25
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    \$\begingroup\$ I see now that you are using the text of bless ("the target can roll a d4 and add the number") to mean someone with bless chooses to use the mod or not and that this will double their chance of landing on the number desired to get their total attack roll to 20. This is clever and and hadn't considered it, so I appreciate the extra detail. \$\endgroup\$
    – Kirt
    Commented Feb 10 at 19:27
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No where in the description does it say "Natural" or "Unmodified" 20. One could take the littoral wording and arrive at a head lopped off rather easily. The description says attack roll of 20. I know as a DM I use "Nat" 20. I had a long discussion after the game session one night about it. A house rule to clarify the wording was created as a result. My answer to the adamantine armor stopping a vorpal hit is NO. As the vorpal hit is not a part of the critical hit/damage mechanic.

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    \$\begingroup\$ Littoral refers to coastal, you may have meant literal. Beyond that, you seem to have repeated the same answer as Adam's, with less support. Welcome to RPGSE. The tour, help center, How to Ask and How to Answer. \$\endgroup\$ Commented Oct 25, 2021 at 12:49

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