Artillerist crossbow

This weapon was used by the great dwarven sappers of the Grayhawk mountains. They used these weapons to break sieges and disrupt the Drows battel formations. Only a hand full of the powerful weapons made it out before the great collapse of their kingdom. 

The weapon can switch between fire, acid, cold, lighting damage as a bonus action. It also has four Viles of level 4 fireball in the lower part of the crossbow aka the fireball launcher. It takes your action to fire them and the Viles are extremely rare to find or make. Also, you can change the damage type of the fireball as bones action. 

You can change the lower assembly out for a flamethrower instead of fireball launcher. The flamethrower holds four vile of liquid fire. flamethrower does 5d8 fire damage in a line 15 ft long. A creature saves a Dex 12 it only takes half damage but is still on fire. It lights anything on fire it will out in 1 min or take its action to put out the flames. If the item is flammable, it will keep burning till it's put out.

Proficiency with a Heavy Crossbow allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Damage: Fire, Damage: Acid, Damage: Lightning, Damage: Cold, Bonus: Ability Checks, Ammunition, Heavy, Loading, Range, Two-Handed, Push