The heroes needed a sacrifice to despair in order to grant the hero, Yuno, with the strongest Light (not sure if they knew the sacrifice gets the polar opposite, Darkness). They banish him, after making him "powerless," somewhere monster-infested to die.
His new power allows him to deal damage proportional to the Darkness power's level (i.e., 1dps @ lvl, 100dps @ lvl 100) to those he classifies as an enemy within a certain distance from him (10 meters, I think). It doesn't limit itself to living beings, inanimate objects and concepts like projectiles and magic respectively can take damage and be destroyed. If the target doesn't have any protective wards or the like, said target won't know they're taking damage till the final second where their HP drops to 0. He can bestow upon his "subordinates" a power befitting of Darkness, making them incapable of disobedience and hurting their master,, directly or indirectly. As far as I've read, the vanguard Riotte and his corrupt rape-happy goons will be the first to be offed.
Irina will be next and it's a satisfying punishment.