Does the barbarian deal one more die of damage with the 9th-level Brutal Critical feature, or two?
I had a massive fight with GM over this – him going for the one extra die of damage, me for two.
The barbarian's Brutal Critical feature states:
Beginning at 9th level, you can roll one additional weapon damage die when determining the extra damage for a critical hit with a melee attack.
While the rules for a critical hit state:
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. To speed up play, you can roll all the damage dice at once.
For example, if you score a critical hit with a dagger, roll 2d4 for the damage, rather than 1d4, and then add your relevant ability modifier. If the attack involves other damage dice, such as from the rogue’s Sneak Attack feature, you roll those dice twice as well.
So it would seem like the barbarian actually rolls 2 more dice.
What's more, it is curious that some magic weapons, like for example sword of sharpness, say that when you roll a 20 on the attack roll you deal bonus 14 damage, not 4d6. Those features add bonus dice, only their condition is for a critical hit to occur. Am I understanding that correctly, or is my DM right, and why?
The same question can be applied to a vorpal sword dealing additional 12d8 on a critical against a creature that doesn't have or doesn't need a head, etc., while the description of the item states that it deals "an extra 6d8 slashing damage".