Line Launcher

The Line Launcher is a grapnel device used by Batman, Batgirl, and Nightwing to get across large gaps or distances that couldn't be normally traversed. It was an alternative to the Grapnel Gun. It could only be used to traverse horizontally, not up or down.

Arkham Origins Incident
The prototype Line Launcher was seen at Batman's Workbench at the Batcave. As it was still in production, Batman utilizes the Remote Claw instead. Batman most likely used the technology of the Remote Claw to complete the Line Launcher after the events of Arkham Origins, as the Line Launcher was clearly missing a few pieces when it was viewed in Arkham Origins.

Arkham Origins Blackgate Incident
It seemed that the prototype was finished right after the incident, as Batman used it during another Blackgate Prison breakout. Batman more than likely used the pieces from the Remote Claw that he recovered from Deathstroke to finish it.

A Matter of Family Incident
Batgirl utilized the gadget during her attempt to rescue her father from Joker and Harley Quinn at Seagate Amusement Park.

Arkham Asylum Incident
After Batman took on two Titan Henchmen at the Botanical Gardens, he summoned the Batwing to drop off a metallic, cylindrical casing. When Batman opened the metallic casing, out popped the Line Launcher in which he used to get across large gaps around Arkham Asylum. In particular, it helped him through the Sewers. The Line Launcher was also very important in the later stages of the Riddler's challenges.

Arkham City Incident
In Batman: Arkham City, the Line Launcher was given to Batman by Robin. Batman could fire the Line Launcher in a different direction while he rode it. There was an upgrade that allowed Batman to flip onto it while he rode it to use the rope as a vantage point. It would disappear after he dropped off of it.

During the Mr. Freeze Boss Battle, Batman might deploy the Line Launcher to initiate a Takedown on him. However, Batman could only attempt this takedown once as with any other takedowns within this encounter, for Mr. Freeze would recognized the attack and rendered it obsolete.

Nightwing also utilized the Line Launcher, with identical functions to Batman's Line Launcher.

Arkham Knight Incident
Batman began the incident with the Line Launcher and it was functionally identical to how it worked in Arkham City. It could also be used while gliding, as long as there was a wall on both left and right sides of Batman. Batman may employs this tactic to set up a vantage point for a Fear Multi-Takedown on unsuspecting Arkham Knight's Militia during the Own the Roads mission. Throughout the night, Batman must utilize this gadget in order to solve many riddles which was created by The Riddler.

Nightwing no longer had the gadget, but the playable Batgirl could use it at will in her respective mission.

Trivia

 * In Arkham City and Arkham Knight, Batman could choose to do a Line Launcher Takedown on a henchman while riding the Line Launcher and knock him unconscious, whereas bumping directly into him would leave him stunned on the ground temporarily.
 * Batman used a device with a similar function to the Line Launcher in Batman Returns, though it possessed a far more simplified design.
 * When the Remote Claw was upgraded to the Line Launcher, it lost some of its original functionality, as the Remote Claw was more combat oriented, such as being capable of Vertical Takedowns from a distance. In retrospect, the Line Launcher was more movement oriented, and was capable of creating a line between any two points, instead of only vantage points.
 * The Line Launcher is one of only five gadgets to appear in all Arkham games so far: the others are the Batarang, the Remote Control Batarang, the Explosive Gel and the Batclaw.