Everything, from his creepy voice and appereance, to his demeanor is already threatening, but watch his Villainous Breakdown and it will seriously give you shudders. He transforms into a hollow-eyed, dead-looking, grotesque version of himself and strangles Therru by pumping up his arm.
Cob: I don't want to die!
Narm aside, the moment at which Arren slices off Cob's arm is nightmare-worthy. Especially when it transforms into black goo and Cob reattaches it.