The Wrist Dart is a gadget used by Nightwing in Batman: Arkham City. They are wrist-mounted projectile launchers concealed on Nightwing's forearm capable of firing a dart. The dart injects its victim with a chemical designed to make them fall unconscious.
Shooting the enemy in the neck or head rendered them absolutely unconscious. However, it simply stunned them if it was shot at another body part.
Nightwing had either abandoned their use by the time of Batman: Arkham Knight, or simply didn't have them with him.
The player had a limited supply of Wrist Darts which can be equipped by holding L1/LT/RMB (PS3/XBOX360/PC) and thrown by pressing R1/RT/LMB (PS3/XBOX360/PC). You can also initiate a "Quickfire Wrist Dart" throw by just tapping R2/RB/L (PS3/XBOX360/PC) and it hit the closest or most important target.