darkness_devil
The Darkness Devil is a devil that embodies the primal fear of the dark. It serves as the secondary antagonist of the International Assassins arc.
The only contract with it seen so far is with Santa Claus (Chainsaw Man), in exchange for bringing the heart of Denji (Chainsaw Man) to Hell, Santa Claus (Chainsaw Man) gained a small portion of the devils flesh which allowed the assassin to manipulate their body in darkness and amplifies their total power.