Please, emphazise silence in his kit in some form or another.
Yeah, creepy grunts and voices can be scary but the correct use of silence can be just as scary if not more (and can even give a character an eerie aura).
Just imagine a situation in which the current Nocturne ults, the screen goes pitch black and every sound fades away. You can still use abilities, but you can't hear a thing, only silent whispers coming from the distance. You anxiously stand still for a couple of seconds waiting for the monster to emerge from the shadows wondering if you are its prey or not . This would be quite fitting from a thematical standpoint, if you ask me.