Source: Bigby Presents - Glory of the Giants

Weapon (Musket), Very Rare (Requires Attunement)

This magic weapon is a triple-barreled bronze musket. You gain a +1 bonus to attack and damage rolls made with it. It requires no ammunition, its damage is radiant instead of piercing, and it doesn’t have the loading property. The base of the weapon is emblazoned with the light rune.

Additionally, while attuned to the weapon, you can cast Dancing Lights from the musket at will.

Invoking the Rune. As an action, you can invoke the weapon’s rune to cast the Sunbeam spell (save DC 17) with it. Once the rune has been invoked, it can’t be invoked again until the next dawn.