Boom Hammer (5e Equipment)
From D&D Wiki
Martial Melee Weapons
"A trick weapon used by the old hunters, and crafted by the workshop heretics, the Powder Kegs. A giant hammer equipped with a miniature furnace. When ignited and fired, it emits a volley of flame that explodes furiously upon impact. Crush the beasts, then burn them - the brute simplicity of the Boom Hammer was favored by hunters with an acute distaste for beasts."
Trick: This weapon can be transformed as a bonus action.
Transform: This hammer is equipped with a miniature furnace. Taking a bonus action before your attack to "light" it, your next attack deals an additional 1d6 fire damage before resetting to the normal state.