One of my players brought this to my attention today and while it seems entirely legit, I wanted to present it to all of you.
Great Weapon Master has 2 bullet points, the most popular of which allows you to take a -5 penalty to attack with a Heavy weapon you are proficient in and deal an extra 10 damage if you hit.
The very first bullet point is as follows
"…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…"
Nowhere in this first bullet point does it mention the melee weapon must be 2 handed or Heavy; only that it be a Melee Weapon.
Enter Assassinate from the level 3 Assassin Rogue
"…in addition, any hit you score against a creature that is surprised is a critical hit…"
So the question is, if the groups Assassin effectively gets the drop on a group of enemies and surprises them, has a chance to act before them in the initiative while they are still surprised, and hits them with a 1 handed Melee Weapon attack; this allows the assassin to auto crit due to Assassinate.
Would this then trigger the first bullet of Great Weapon Master, allowing the Assassin to make another melee weapon attack as a bonus action because he scored a critical hit; which would result in a second auto crit because the enemies are still surprised and still have not yet acted in the initiative because it is still the Rogue's turn?
Best Answer
Yes, it would trigger the bonus action and it would be also a critical hit.
The feat Great Weapon Master calls for a critical hit, not a natural 20. This is important because Assassinate is not the only ability that modifies the critical range, e.g. Improved Critical from the Fighter also modifies the critical range. Also, this bonus attack would be a critical hit. It may seems OP but bear in mind that melee rogues usually wield two weapons for the benefit of Two-weapon fighting. Furthermore, since you only have one bonus action both features are incompatible.