Can a Barbarian choose which weapon die to add for his Brutal Critical?

Question

Our Barbarian has a Flame Tongue longsword.

While the sword is ablaze, it deals an extra 2d6 fire damage to any target it hits.

When the Barbarian crits, he doubles all the weapons damage dice, and then adds a few more due to Brutal Critical. In other words, assuming first rank of Brutal Critical (for simplicity), a crit would give him 1d8+2d6 (regular damage) + 1d8+2d6 (critical damage) + X (brutal critical damage). If I understand correctly, X will be a single die, like 1d8.

However, we have argued that maybe it could be 1d6 (if the Barbarian wanted to deal fire damage instead of slashing damage). We are also not positive if this is correct, or if maybe we should be adding 1 of each die, so X would be 1d8+1d6.

When weapons have different damage dice, how does Brutal Critical interact with them?

Answer 1

Following the discussion here, where a weapon's extra damage also counts as weapon damage dice, then

You can choose either 1d8 slashing or 1d6 fire extra damage for your Brutal Criticals.

Crawford supports this idea with the tweet:

Savage Attacks: add 1 of a weapon's dice to a crit. again. Frost brand is a wpn. w/ 1d6 cold as 1 of its dice

For Frost Brand, which has a very similar wording to Flame Tongue, you can add the frost damage dice to the Orc's Savage Attacks, not only the sword's damage. Therefore, we can extrapolate and assume Flame Tongue behaves similarly with the Barbarian's Brutal Critical.

Answer 2

The barbarian* can choose which die to add.

Weapons (PHB, page 146 (emphasis mine))

The Weapons table shows the most common weapons used in the worlds of d&d, their price and weight, the damage they deal when they hit, and any special properties they possess.

Following that, a regular longsword shown on the equipments table deals 1d8 damage when they hit. The weapon die is 1d8 slashing. The Flame Tongue, when activated deals 1d8 slashing + 2d6 fire damage when it hits. So the weapon die for an activated Flame Tongue is 1d8 slashing + 2d6 fire.

Further assisted by the wording of Flame Tongue (emphasis mine):

While the sword is ablaze, it deals an extra 2d6 fire damage to any target it hits.

Note it is not an additional effect that is dealing the extra damage but the sword itself. That is the same wording as the quote from Weapons (PHB 146). Saying "The sword deals 1d8 + 2d6 when it hits." means that it's the weapon die.

The Brutal Criticals feature specifies one die instead of saying roll weapon die one additional time, or something similar, that eliminates rolling one of each type. So a barbarian that crits with an activated Flame Tongue can choose either 1d8 slashing or 1d6 fire to add as extra damage from Brutal Criticals.

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*: Blake Steel pointed out the barbarian may not be the one choosing which die Brutal Critical adds.

[...]do you have any evidence to back up that they can choose which damage dice...

Brutal Critical says:

You can roll one additional weapon damage die when determining the extra damage for a critical hit with a melee Attack.

There is no RAW about what happens if said weapon has more than one type of die. Since someone has to choose which die is added, in the absence of anything contradicting we can say the barbarian can choose the effects of his own feature.

Answer 3

The fire damage is not weapon damage, but they are all part of the attack damage.

Brutal Critical says "you can roll one additional weapon damage die" (you get more dice at higher level).

The rules for critical hits say "roll all of the attack's damage dice twice and add them together"

So, RAW, you do not have a choice of which dice to add for Brutal Critical. In your example it would be the 1d8 (slashing) for the weapon die