The Magnetic Gas-Powered Grapple Gun is a grappling hook weapon designed by Lucius Fox at Wayne Enterprises that was acquired by Bruce Wayne when he began building his new identity as Batman. Magnetic and tested for 350 pounds, the gun is holstered on the back of the utility belt on his Nomex survival suit.
After being introduced to the Applied Sciences Division, Bruce Wayne bought the grapple gun alongside a Nomex survival suit and the Tumbler. He later used the gun to get on a rooftop after the Scarecrow set him on fire and used it to escape a bunch of prisoners under the influence of the the tormentor's fear toxin.
Batman used the grapple gun multiple times during his final confrontation with the Joker, mostly notably to save a few hostages and the Clown Prince of Crime himself after knocking him over a ledge of the Prewitt Building.
Batman used the grapple gun against a group of robbers led by Bane who were escaping on their motorcycles with some hostages from the Gotham Stock Exchange, pulling one of them off of the vehicles and confronting him.