Love the idea of Outsider being higher but not the highest deity. And who is the highest one, you may ask me? The Void! Just the eldritch god, as old as the time itself, lonely and eager to communicate. But along with that Its only touch is deadly for mere mortals, as It's a pure havoc that will absorb the very soul of the one It's communicating with. 

Corvo describes the Void as an “endless cold”, Daud says that the Void feels like “if you cried for help but no one answered”. And for them it's true but for the eldritch one it's the form of communication. A love language. It lets you roam inside freely and It expects you to let It do the same, let It roam inside of you. Cup your soul in Its improvised hands to hold just for a moment, to study, to feel your emotion, to live through your memory. To show you some love in Its term, to do the softest thing It can do!

And who knows maybe this communication was the main goal of whoever created Outsider. But as time went by, this goal was lost and forgotten. So here It is and here is Outsider, both so old and not even sure how much of them was inherently theirs and how much they have absorbed from each other.