GUYS WHAT IF ITS NOT FNAF, WHAT IF ITS SCP CONTAINMENT BREACH
My headcanon for how the Author became the Host basically cribs notes from SCP Containment Breach.
My thought is the Author/Host was held in an SCP-style facility, and escaped during a prison break… but not before an inmate gouged out his eyes. That would also explain the straitjacket-style trenchcoat he likes to wear; it’s what he was strapped into when he escaped.
Also? Dark was the one who found him, which is why he’s willing to work for the demon (as implied in Markiplier TV). He owes Dark for saving him.