This legendary turret gun is located in the demon door in Wriathmarsh.

Just bring about ten people with you and transport them to the drown farm, kill the Banshie and run to the door.

He will say a speech, the door will open and there will be a house.

Up the stairs is terracotta room and his final journal entry.
The weapon itself is through the back door of the house and through a gate.

The gun is all the way in the back of the army in a chest.

Villagers will be afraid of it, even when it's not out.