This weapon is found within the massive floating raider encampment, Libertalia. It is located at the top of a lootable wooden crate within the captain's cabin at the top of the compound. The crate is located immediately across the room from the door, beside a burning lantern.
Behind the scenes[]
While the French is not perfectly translated (translating more directly to "The Terrible Gun," with "terribles" unnecessarily pluralized), the name, along with the gun's role as a unique combat shotgun with higher damage but higher recoil and also the fact that both are found in raider strongholds, all make it a likely reference to The Terrible Shotgun from Fallout 3.
The name of the weapon is a reference to the French novel and movie Les Enfants Terribles.
Gallery[]
Le Fusil Terribles in the Libertalia captain's cabin