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?
Best Answer
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.