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Great Weapon Master grants the option to take a -5 to your attack roll in order to get a +10 bonus to damage. Due to bounded accuracy, that's a pretty big hit on your chances that's not easy to overcome.

An upcoming adventure I'm planning will feature as a reward a special weapon that is so heavy, it has a -5 to hit and a +10 damage by default, but I'm somewhat concerned whether it might cause problems if I allow this weapon to stack with the Great Weapon Master feat.

Will anything in the game break, or will the wielder of a Heavy Weapon become too powerful, if they are allowed to take a -10 to attack rolls for a +20 damage bonus? Or will they just be unable to hit anything outside of a lucky roll?

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  • \$\begingroup\$ Is this weapon magical? \$\endgroup\$ – András May 18 '18 at 22:00
  • \$\begingroup\$ @András no, just very heavy. \$\endgroup\$ – Erik May 18 '18 at 22:01
  • \$\begingroup\$ @Jasen please don't answer in the comments. \$\endgroup\$ – Rubiksmoose May 19 '18 at 1:07
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The weapon is not overpowered

The great thing about GWM is that you don't have to use it. When you have disadvantage, or simply an enemy with really high AC, you can attack without it.
You have a choice.

With this weapon, GWM is always on. The biggest benefit is not that you can double it, but that you can spare spending an ASI on the feat.
But you need a second weapon for the cases where you would not use GWM.

Power level

An ASI is about as good as a simple, uncommon +1 weapon if you are Str primary. ASI = feat, so this weapon is worth about an uncommon item.
Except you have to replace it against hard to hit enemies, and non-magical resistant enemies.

Anyone who has GWM already is much better off with a +1 weapon, making this weapon too weak for uncommon rarity.

If you don't want your players to replace it at the first opportunity, make it a +1 magical weapon.

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Doubling GWM reduces average damage output.

I did some math to calculate how having a doubled GWM feat affects your chances to hit and your average damage output. I assumed that the PC was a single-classed fighter, starting with 18 STR and using their first ASI to get to 20 STR, and I assumed the enemy was always a monster whose defensive CR was equal to the PC's level. I also assumed that your homebrew weapon dealt 1d12 damage.

With these calculations, the necessary minimum roll to hit is remarkably consistent throughout the levels.

Without GWM, the fighter needs to roll at least a 7 to hit at early levels, and needs to roll an 8 to hit at most levels above level 9. Their average chance to hit on any given attack from levels 1-20 is about 63%, for an average damage output of 7.1 damage per round (11.35 dmg per hit).

With GWM, the roll to hit increases to to 12-13, and the average damage rises to 21.35 dmg per hit. The chance to hit drops to about 38%, but the increased damage outweighs that, so the average damage per round rises to about 8 damage per round.

With double GWM, the roll to hit further increases to 17-18, and the damage per hit rises to 31.35. However, the chance to hit drops to 12.75%. At this point, the drop in chance to hit outweighs the increase damage, and the average damage per round drops to 4 damage per round.

Even when the PC has advantage, doubling up on GWM significantly reduces their average damage per round.

These statistics mean the weapon is a little better than a +1 weapon in gameplay.

So what's the effect of these statistics in actual gameplay?

First, nobody will ever want to double up on GWM, since it cuts your average damage output roughly in half. I can maybe imagine a desperate situation where the PC would want to go for broke for extra damage, but that will be rare.

Second, you're granting half of the Great Weapon Master feat. As we can see from the stats, the boost in damage per round is approximately the same as a +1 weapon, at least under the assumed conditions. However, a player who would have taken the GWM feat might take an ASI for their attack stat instead, which is equivalent to a further +1 weapon, which you should consider.

Finally, hidden in these average statistics is the fact that this weapon will cause your combats to become more swingy. Instead of dealing a steady drip of damage, this PC will be useless most of the time, and suddenly deal a ton of damage every once in a while. As a DM, I find that rare occasions of "good" luck are much harder to deal with than more predictable situations. You'll have to decide how much swingy-ness you and your players are willing to tolerate.

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