[RPG] apply the Great Weapon Master feat’s second benefit (-5 to attack roll, +10 to damage roll) to the bonus-action attack from the first benefit

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Say I trigger the bonus-action attack granted by the first benefit of the Great Weapon Master feat (PHB, p. 167), either by getting a crit or a killing blow. Can I then apply GWM's second benefit (-5 to attack roll, +10 to damage roll) to that bonus-action attack?

As a barbarian at level 5 (Extra Attack) , this would mean that I could use this feature a total of 3 times in a turn — that's some heavy damage.

Best Answer

Yes, you can take -5 to hit/+10 damage on the bonus-action attack.

The Great Weapon Master feat (PHB, p. 167) reads:

[...] you can make one melee weapon attack as a bonus action.

and

Before you make a melee attack with a heavy weapon that you are proficient with, [...]

So whenever you make a melee attack (with a heavy weapon that you are proficient in), including one provided by the feat, you can take the - 5 to hit for the +10 damage. This can be done any number of times - so yes, your barbarian could do it on three attacks (attack, extra attack, bonus-action attack) in a turn.

Yes, you can move before taking the bonus action attack

No timing is specified

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.

The phrasing here says nothing about the precise timing of the bonus action. It can be simply read as saying that after the condition is met (critical hit or reducing to 0 hp) that you are granted the ability to make a bonus action attack. It never says the bonus action must be taken "immediately" or anything along those lines so the timing of the bonus action attack is not restricted at all.

You choose when to take a bonus action during your turn, unless the bonus action’s timing is specified

The rules for when you can take a bonus action say that if the timing is not specified, then you can choose when to take it. Thus, there is no reason why you would not be able to choose to move before taking the bonus action attack.

The rules explicitly allow moving between attacks

If you take an action that includes more than one weapon attack, you can break up your movement even further by moving between those attacks. (PHB 190)

Even within a single Attack Action, the rules allow one to move between attacks. It absolutely makes sense that they would allow it between an Attack and a bonus action attack for example. After all, even if the bonus action isn't considered to be part of the Attack action (or whatever triggered the GWM bonus attack condition), it is indeed a weapon attack and thus moving between them seems like an incredibly reasonable interpretation.

Jeremy Crawford has agreed this is the intended interpretation

Jeremy Crawford has clarified this exact issue via Twitter and agrees with the interpretation above:

The intent is that you could move before taking the bonus action in the Great Weapon Master feat.

[RPG] way to make the Elven Accuracy feat work with the second benefit of Great Weapon Master

The most optimal way to do this would be to play a Half-elf Hexblade.

Drow (PHB) & Shadar-Kai (UA Elf Subraces) come in second (+1 to Cha vs Half-Elf's +2), and other elves are still viable (just a little behind on Cha). You can start using a heavy weapon once you reach 3rd level in Warlock with the Hexblade Patron.

Hex Warrior (XGtE 55,56)

Proficiency with martial weapons (as well as medium armor and shields)
Select a single weapon which is not two-handed and use Charisma for attack and damage. This extends to your pact blade, regardless of its type.

Pact of the Blade (PHB 107)

At 3rd level you can choose to summon any melee weapon as your Pact Blade. At this point you may choose your heavy weapon and attack with Charisma.

Also note that your attack cantrips such as Eldritch Blast and Shocking Grasp would benefit from the reroll as well.

Here are some ideas that come close but I don't think quite work:

3rd level Paladin with the Oath of Devotion
- Sure, you add Charisma to your attack. That probably doesn't count as using Charisma. Let's call it a bonus. Note that you don't add Charisma to your damage. (Maybe you can convince your DM that this works. It comes down to what using Charisma actually means in the context of the various rules.)
- Also, this takes an action where you could have been attacking and uses up your Channel Divinity that you only get once per rest.
Kensei Monk with a Bow
- Improvised weapons don't inherit properties from what they really are. Using a bow as a club/staff doesn't give it the heavy property and doesn't make it a monk weapon. In melee, a bow is just a length of wood (not even as good as a staff). (It certainly doesn't inherit the ranged property.)

Honestly, it makes sense that you are having a hard time using a heavy weapon without using Strength. This is part of the game balance that 5e sets up. If anything Hexblade/PoB breaks this balance by using Charisma with any weapon. Don't be surprised if you see Hexblade as the go-to for getting around balancing rules.