Does a Champion fighter automatically hit someone wearing Adamantine Armor if they roll a 19 (or 18 at higher levels) on the attack roll?

Question

Without adamantine armor, a critical hit is an auto-hit regardless of AC. Though Jeremy Crawford once tweeted that a Champion fighter with the Improved Critical and Superior Critical features auto-hits when the attack roll lands on a natural 18 or 19 because it's a critical hit, unlike in the PHB where the nat. 20 auto-hit is an entirely different wording.

Would a Champion fighter who rolls a 19 (or an 18, at higher levels) on the attack roll auto-hit against someone wearing adamantine armor? (The same could apply to any other class/subclass with an identical feature.)

I'm assuming it doesn't, since it would take away its critical hit status, which is the only reason it auto-hits.

Answer 1

Yes, the attack still hits

It follows from the wording of Adamantine Armor:

… any critical hit against you becomes a normal hit.

Improved critical hits are critical hits. Adamantine Armor turns critical hits into normal hits explicitly; not into misses.

Answer 2

The attack hits, but becomes a normal hit.

At the point where the effect of adamantine armor comes into play, the wearer has already been hit. Features and effects do what they say they do, and adamantine armor does not provide a way to convert a hit into a miss.

Improved Critical hits on a roll of 19.

The feature states that attack rolls of 19 hit.

...your weapon attacks score a critical hit on a roll of 19 or 20.

It happens to be a critical hit which has other implications, but the important part is a roll of 19 hits for a fighter with improved critical. There's no mention of AC or other modifiers affecting this feature. It's a hit.

Adamantine armor removes the criticality from a hit.

Adamantine armor mitigates the subsequent implications of the criticality of the hit. Usually, this is the extra damage dice, but there are other features that operate on a critical hit which this armor also mitigates.

... any critical hit against you becomes a normal hit.

The armor converts a hit to a hit. There is no implication that a hit can be converted to a miss. A hit becomes a different kind of hit.

It's not by virtue of being critical that the attack hits.

Player's Handbook p.196 describes the impact of a critical hit which is to roll extra damage dice. Nothing about being a critical is guaranteeing the hit.

Rolling a 20 is what guarantees the hit.

The PHB section on Rolling 1 or 20 does not affect Improved Critical. The section (p.194) states a roll of 20 hits. It includes additional clarification to head off any confusion that might arise from trying to figure in attack modifiers or AC:

If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target’s AC.

It's a useful additional explanation to make it crystal clear, but the statement still stands. On a attack roll of 20, the attack hits. This is the same statement that improved critical makes about a roll of 19. On a roll of 19 the attack hits.

Sample Walk-through

1. Alice, a fighter with improved critical, attacks Bob with their longsword and rolls a 19.
2. DM indicates this is a critical hit.
3. Bob is wearing adamantine plate armor, which changes the hit from a critical one to a normal one.
4. Alice rolls normal damage.

It's important to note at step 2 there was no AC calculation done. No checking the modifiers on the roll such as proficiency and strength. No need. Improved critical makes a roll of a 19 a hit.

Answer 3

It Depends

AKA: the very worst kind of answer.

From your comments I think what you are really asking is "If a critical hit on adamantine armor would have missed except for being a crit, does it still hit?" If I am wrong on reading your question let me know. But assuming I am not, then the answer is the same for any other attack roll that you make. You hit the enemy if your roll beats their AC.

For a concrete example, lets say you roll a 19 and have a +6 to hit. Your total attack roll is a 25 and the 19 gives you a critical attack. Against most enemies all you care about is the second point, since a crit is an auto hit. Against an enemy wearing adamantine armor (or who ignores criticals in similar fashion) your attack is just a normal attack and not a critical hit. So treat it like a normal attack and compare your attack roll (25) versus the target's AC. If their AC is 30 then you are out of luck, if it is 24 then you get a normal amount of damage on them. Easy-peasy.

Narratively you can think of the adamantine armor as being so exceptionally durable that it lacks any weak points most other armors have. Critical attacks normally hit those weak points, representing something like getting a dagger through an armored joint or a sword between a dragon's scales. Adamantine armor is so tough that you essentially can't even get in a lucky hit.