[RPG] Great Weapon Master Critical Hit Resolution

critical-hitdnd-5e

My apologies if my unfamiliarity with the rules makes this an easy question to answer…

I am considering taking Great Weapon Master, but was curious how this would affect my critical hits. I am currently a level 20 Half-Orc Barbarian, meaning that when I land a crit, I can add a bunch of extra damage dice (I believe I am able to add 3 extra dice from Brutal Critical, and Savage Attacks allows me to add another extra die).

However, how are critical hits affected by Great Weapon Master? If I take the -5 penalty before rolling, and then roll a nat 20, will the critical still hit, AND will I then get to add the extra +10 to damage dealt? Or would the roll become a 15, thereby negating the critical hit altogether?

Great Weapon Master states:

Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attackā€™s damage.

This becomes even more important given the other feature of Great Weapon Master:

On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.

So how does Great Weapon Master affect critical hits?

Best Answer

Your critical hits are not affected at all!

If you roll a 20 on an attack roll, meaning the die itself shows a 20, it is always a critical hit, as explained on PHB p. 194:

If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. This is called a critical hit, [...]

The GWM damage bonus is also not affected, because only the damage dice are doubled. You can still add your brutal critical bonus dice, but the +10 on damage does not change, because it is not a die.