Source: Bigby Presents - Glory of the Giants

Weapon (Pistol), Very Rare

This magic ranged weapon is a flared pistol with the storm rune engraved along the barrel. You gain a +1 bonus to attack and damage rolls made with it. It requires no ammunition, its damage is thunder instead of piercing, and it doesn’t have the loading property.

Invoking the Rune. As a bonus action, you can invoke the weapon’s rune to launch a ball of energy to a point you can see within 30 feet of yourself. The energy then detonates into a 10-foot-radius sphere of turbulent wind and thunder centered on that point, and each creature in that sphere must make a DC 14 Constitution saving throw. On a failed save, a creature takes 3d6 thunder damage, and it can’t take reactions until the end of your next turn. On a successful save, a creature takes half as much damage only.

Once the rune has been invoked, it can’t be invoked again until the next dawn.