The fifth world.

The Void is fivefold. Co-eternal. This is the nature of the Void.

What substance is the Void?

We do not yet know. We do not yet know if it is impossible to know. The Void likely transcends the distinction between what is and isn't a substance.

What is the Void's will?

The Void's will is a Rin, a beautiful bird with iridescent feathers and a little Void as its crown. This is, however, an aspect of Their will it is possible to know.

What does the Void sound like?

The Void sounds like your mother's voice.