It stays as 6d8 but x2

That's false, crits roll all rolled dice twice for an attack.

I’m gonna ask my DM for this for my birthday.

i love this including it in a caimpain

cool

Sheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeesh

69 max damage… nice

If a creature is immune to slashing damage, how can it suffer 6d8 slashing damage?

[deleted dumb post]

Quote from Fallenstar004 >>

’Twas brillig, and the slithy toves

      Did gyre and gimble in the wabe:

All mimsy were the borogoves,

      And the mome raths outgrabe.

 

“Beware the Jabberwock, my son!

      The jaws that bite, the claws that catch!

Beware the Jubjub bird, and shun

      The frumious Bandersnatch!”

 

He took his vorpal sword in hand;

      Long time the manxome foe he sought—

So rested he by the Tumtum tree

      And stood awhile in thought.

 

And, as in uffish thought he stood,

      The Jabberwock, with eyes of flame,

Came whiffling through the tulgey wood,

      And burbled as it came!

 

One, two! One, two! And through and through

      The vorpal blade went snicker-snack!

He left it dead, and with its head

      He went galumphing back.

 

“And hast thou slain the Jabberwock?

      Come to my arms, my beamish boy!

O frabjous day! Callooh! Callay!”

      He chortled in his joy.

 

’Twas brillig, and the slithy toves

      Did gyre and gimble in the wabe:

All mimsy were the borogoves,

      And the mome raths outgrabe.

- Lewis Carroll, The Jabberwocky

 this poem takes on a whole new meaning with the jabberwock being added.

I got a question; So the enemy Has legendary actions .... they use them up .... no more legendary actions .... is the Nat 20 decap now in play or is it if the creature has/had legendary actions it negates the decap?

Quote from Vman9484 >>

I got a question; So the enemy Has legendary actions .... they use them up .... no more legendary actions .... is the Nat 20 decap now in play or is it if the creature has/had legendary actions it negates the decap?

 A creature still has Legendary Actions even if it has used them all up for the turn.

i made a vorpal boomerang

well that wasn't supposed to happen because Zariel has legendary actions

RAW the 20 doesn't even need to be natural because it doesn't specify d20 roll. It says attack roll. 
The weapon states "roll a 20 on the attack roll" and in the attack rolls section of the phb, it states "To make an atta⁠ck roll, roll a d20 and add the appropriate Modifiers"

Example: A +13 to hit would require exactly 7 on the d20 assuming no other bonuses such as bless.
The only way to get the extra damage doubled for a crit would be to have them be auto crit via paralysis or some other form of auto crit that still requires a roll. You could also somehow get a +0 to hit with it (not proficient + bane? Or be a low level with great weapon master and bad strength)

RAI: Probably just a nat 20 like everyone else says

I would like to address some common misunderstandings here -

• The sword's special "Vorpal" ability (the second paragraph above) only happens when you roll a natural 20 on your attack roll. The rule does not mention anything about a critical hit. Natural 20 only.

• If a creature is immune to critical hits, the ability still triggers on an attack roll of natural 20.

• If the creature is immune to the effect but you roll a natural 20 to attack and this does count as a critical hit, both the initial weapon damage and the extra 6d8 are doubled, as per page 196 of the Players Handbook (emphasis mine)-

"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."

• If a creature is immune to slashing damage, they are immune to the automatic head loss and both the initial weapon damage as well as the extra 6d8, as both are slashing damage. While the sword does ignore resistance to slashing damage, it does not ignore immunity to slashing damage.

Does it count if I get a 17 but I have a +3 modifier, does it still decapitate the monster? 

I know it's been 4 years.. but If I were to pick, I would double the damage of the strike, so say it was a greatsword at the base, so it takes the 2d6 doubled because of the crit, and the extra 6d8. 

cool colors no wonder it is the moster OP wepon

Stays 6d8