Because Void's design hadn't been finalised before hand, I put forth a few ideas to Yagami Yato.
The description I was given for Void was "Genderless (as much as a character can be), greyscale, almost sickly/dead pale skin, with maybe black hair"
Very often it's common for artists to do multiple concepts (sometimes up to 25!) just for 1 character. However depending on your boss/client, sometimes it's better to give less. In this case, it was better to give a more concentrated set of designs.
So I gave these three, each having a very different feel, and different reasoning behind the design choice.
#1) I wanted to play with the idea of liquid, the idea of water reflections, of black goo that just gets everywhere, almost infecting whatever it would touch. I also liked the idea of the hair being like a Void, so i threw in the eyes and mouths to make it somewhat unsettling.
#2) was playing on shattered glass/mirrors, and the idea that cracking them brings bad luck, and all the lore around how evil things can come through mirrors. I also made Void's tentacles separate, more like a tail attached to the body.
#3) i started to look into...eldrich horror kind of artworks, a lot of Dark Souls/Bloodborne monster concepts I looked at. So I liked the idea of something that seemed somewhat normal, but parts of it made you double take a little.
As for the eyes, I threw a bunch of different options out, because Void has no gender, I had the options to play with both feminine as well as masculine eyes, as well as more quirky unusual eyes due to Void not being human. However it was obvious I wasn't going to head towards rounded eyes, as round eyes tend to bring across kindness, and that's far from what Void is.
I like to believe that this "form" that Void shows is just to not scare Caspin too greatly, I imagine he has a true form that is much more horrific!