The Master Assassin, first name Devon, was the head of the Society of Assassins.
When Curaré's first attempt to assassinate District Attorney Sam Young failed, she reported back to the Society by video link. The Master reprimanded her, and reminded her of the price of failure, which was execution.
When Curaré was finally defeated by Batman, the Master immediately ordered her killed, and the first attempt came even as she was fleeing the GPD after her capture. However, she turned the tables and hunted down her fellow assassins, one by one.
Spooked by her success, the Master hired a private jet and spent the next three months traveling, without ever touching down on land. At the same time, he ordered the Society's last surviving member, Mutro Botha, to approach Batman and enlist his help in defeating Curaré. Just as he was giving Botha his final instructions, however, Curaré attacked him, having sneaked on board his plane when it was forced to land, briefly, for a repair.
The Master evaded her initial attacks and kept her at a distance with throwing stars, believing that without her sword she could not strike. Instead, she flicked a gas capsule at his chest, releasing a chemical that wiped his mind clean. His bodyguard was left clutching his body as Curaré depressurized the cabin and jumped out of the plane.