Coalition of Assassins

The Coalition of Assassins (暗殺連合, Ansatsu Rengō; Viz: Assassination Association) is the coalition consisting of the deadliest assassins in the world, all of whom are willing to kill anyone provided the price is right.

Monster Association Arc
After they claim that they were hired by the Coalition's higher-ups, Flashy Flash reveals to Gale Wind and Hellfire Flame that at some point in time, all high-ranking members of the Coalition tasked with killing Flashy Flash have instead been killed by him, with their bodies ending up at the bottom of the ocean.