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 attack’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?