Caltrops

Caltrops are sharp, metal jacks that could be dropped and used as a trap. Enemies trip and fall down when they step on them.

Initiation Incident
Bruce Wayne utilized Caltrops during his Initiation as an assassin by Master Kirigi.

Arkham City Incident
After Catwoman decided to break into Hugo Strange's Vault, she knew that she needed her weapons. She headed over to her apartment to get a spherical metallic ball filled with Caltrops and her Bolas.

Arkham Knight Incident
The Caltrops did not appear in Batman: Arkham Knight during Story Mode, but they were usable in the Challenge Maps.

Trivia

 * Like most DLC character gadgets, Catwoman's Caltrops are real life weapons.
 * In Batman: Arkham City, the player can only use the gadget three times, and they don't replenish unless the Checkpoint is restarted or completed. In Knight, they replenish over time, presumably to balance her lack of climbable ceilings.