"The Trollhammer Torpedo is a masterpiece of dwarven craftmanship!" - DLC Item description.
This ranged weapon fires one large grenade-like contraption in an arc, just like a large grenade-launcher, that will explode upon impact and deal AoE damage. Due to the size of the "torpedos", it takes some time to reload.
High Damage, Sniper, Piercing Bolts, Splash Damage
This is a DLC weapon!
You unlock it by purchasing the "Forgotten Relics"-DLC. After purchase, it will become available to craft in the crafting menu.