The Destructor was a contraption invented by the Half-Blood Prince, for the pupose of destroying Hogwarts.
The Destructor resembled a boat with a tower on it. On top of the tower was a wand that can communicate with a small green fire that was in the possesion of Lord Voldemort. Affixed to the prow of the boat was a wand cannon that could fire the spells: Avada Kedavra, The Imperius Curse, The Cruciatus Curse, and Avada Kedushum.
The only time it was used was when Lucius Malfoy and Wormtail used it to bombard Hogwarts. Once the battle was concentrated inside, they left it and snuck inside. It drifted there until it was promptly sank when Lucius Malfoy and Wormtail (inside the garbage can of imminent doom) were thrown into it.