Can a Zombie wearing Adamantine Armor survive a natural 20 on an attack by a vorpal sword?

Question

If you wonder where a zombie got an adamantine armor, he got it from the poor bloke that got beheaded by a vorpal sword, who thought it was a good idea to wear adamantine armor against a vorpal sword.

Or rather, the bloke was animated by a necromancer who saw the scene and curious if it's a good idea for his zombie army to wear adamantine armors against users of vorpal swords.

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Adamantine Armor turns any critical hit into normal hit.

Zombie has Undead Fortitude trait that gives it a chance to survive non-critical-non-radiant damage.

Arguably, zombie does not need a head to survive, so it can survive beheading by vorpal sword. The only concern is the extra 6d8 slashing damage that increases the Undead Fortitude DC, but at least the zombie army will only be susceptible to radiant damage.

Did I interpret the rule right? Will the poor bloke zombie survive his second beheading?

Answer 1

If the zombie can survive without a head, then this can work but is extremely unlikely

A creature is immune to [the beheading] effect if it... doesn't ... need a head

This means that the attack is simply a critical hit that deals 12d8 + critical +3 weapon damage. This would almost certainly kill a zombie.

However, adamantine armor cancels the critical hit...

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

... making the attack only deal 6d8 + normal +3 weapon damage.

Undead Fortitude let's the zombie have a chance at surviving...

If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken

...but the chance is very low as the vorpal sword does a lot of damage (weapon damage die + ability modifier + 6d8 + 3). It is in theory survivable, but that doesn't make the necromancer's tactic particularly effective.

Answer 2

Yes, but very rarely!

Con save vs 18 or greater (minimum base damage) with Con +3 to saves O.o

(Base 5, +3 magic property, +6d8 slashing, 1d6+ sword, ability modifiers +1 or greater, and proficiency bonus +2 or greater)

In addition the zombie would suffer penalties for not being proficient in armor as well as -2 Dex modifier.

Armor Proficiency - If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can't cast spells.

• Medium armor (excluding Hide) give you 11-13 AC @ 20 speed
• Heavy armor gives you 14-18 AC @ 10 to speed.
• Adamantine armor cost 151 to 2,000 gp.