The Below is a vague term I used to attempt to describe a sort of sub-space that the Outsider uses for travel.

Every reality exists in a space-time bubble. Outside of said bubbles, reality does not exist. Anything that goes outside of said bubble ceases to exist, and to have ever existed.

Something carved complex paths through this nothing,lining the paths with warped spars of tangible material essense. This maze of paths is inhabited by various things that have broken dimensional boundaries, like the Outsider but weaker.

The Eternity Gates are each reality’s ‘connection’ to this void. As I said, nothing not in a bubble or on the paths can exist. The Below is a perversion of this, a space-continuum linked to the Outsider that allows him to traverse the Void must faster than going through the paths.

The Below ‘touches’ many realities when inhabitants are stupid about using certain technologies and punch ‘holes’ in reality causing dimensional fatigue errors. These are literal ‘tears’ in reality and the Outsider can influence beings through said tears even if he can’t come through them.

The Below is not a place where intelligent beings of any kind should go — it would be inimical and deadly even for the Leviathans, much less mortal races.

Because there is no connection.

You’ll have to figure that part out on your own.