Darkness, alternatively known as Chaos, is a mysterious outwordly substance which can infect and manipulate people, as well as empower them.
Darkness is likely generated by the god of chaos, Azrail, and is the reason behind Victor Delacroix's change of heart. After he, Sieg Wahrheit and Siela Riviere had sealed the gateway of chaos, the darkness released by Azrail infected Delacroix, causing him to go berserk and attack Sieg. Siela took the hit for him, and was fatally wounded by Victor's blade. When Delacroix came back to his senses and noticed his deceased lover, he instantly assumed Sieg to be the killer, and vowed revenge. After this event, Victor had been infected with the darkness, which clouded his memories and turned the once virtuous man in a cold-blooded mass murderer. The substance gave him a near immortal body, and was also used by him freely, providing him with many abilities, such as regenerating instantly, telekinesis, power to mess with others' minds, summoning monsters and generating destructive energy blasts.