My Fighter (Battlemaster) Warlock (Hexblade) build character has just acquired a suit of +1 adamantine plate armour.
From the description of adamantine armor (DMG, p. 150):
This suit of armor is reinforced with adamantine, one of the hardest substances in existence. While you're wearing it, any critical hit against you becomes a normal hit.
Is a roll of a natural 20 (which is normally a critical hit) still automatically a hit, despite the critical becoming a "normal hit" because of the armour? Or would the attacker need to exceed my AC in order to score the "normal hit" if (for instance) I cast shield or use the Evasive Footwork maneuver to boost AC?
My AC is 20, so with shield cast it becomes 25; for example, would a goblin with a +4 attack modifier score a hit on a roll of 20 against me? A total of 24 would not be sufficient to "hit" under normal rules, but does the "20 is always a hit" mechanism override this despite the critical being cancelled by the adamantine armour?
Best Answer
Yes, the natural 20 is still an automatic hit
We know that a critical hit is, by definition, an automatic hit regardless of AC thanks to the basic rules for making an attack:
If you score a critical hit, you must have hit.
The armor states (emphasis mine):
By saying "a normal hit" instead of "a normal attack roll" (or something similar) the armor explicitly does not change the result of the attack roll (hit or miss), but rather the severity of the hit (critical or normal).
That suggests the following sequence of events:
By the time the item interferes, we've already established the attack as a hit and the armor doesn't undo that hit; it reduces the severity of the hit.
But What about for Champion Fighters?
Champion Fighters have class features which give them an expanded critical hit range
It's already been established in a related question that these improved criticals are also automatic hits
So, the attack is a critical hit, which means that the adamantine armor makes it a normal hit. So, if you face any creatures with improved critical hit ranges, they will still automatically hit you on a natural roll that falls within their range. However, those automatic hits become regular hits just like a critical hit from a natural 20.
We're almost certain this is Rules-As-Intended
Jeremy Crawford, lead rules designer for 5e, has stated over twitter:
While no longer an official source, Jeremy's tweets still provide an excellent look into the intent of the design. Especially since he made this particular tweet back when his rulings were official. Based on this, we can be confident that this stance is the intended reading of the rules.