Quick Uncle Ardyn Concept!:
Ardyn was never imprisoned by Somnus, instead he escaped his brother and hid away in a cavern.
After a few centuries of living alone, he approaches civilisation once again and realises that Somnus' line has been ruling since his 'death'.
One night, feeling as though he has no other option, he crawls into the city of Insomnia (and what a stupid name that is) with the intention of killing Somnus' descendants. Instead, he stumbles across the nursery of the infant Prince. He spends a few moments watching the infant sleep, and then the child opens his eyes and the rage quickly dissipates. The child is, for lack of a better phrase, adorable. The boy reminds Ardyn of the child he had once imagined himself and Aera having.
So instead of killing off his brother's line, Ardyn becomes a guardian of sorts. He's determined to help this child grow into something better, to do better.
From that day on he appears to every LC child up until the age of 18, at which point he stops his visits, until they have a child of their own. Even then, he takes great care never to be seen by the parent, just the child.
And so it goes on. He spends centuries caring for the children, teaching them and scaring away their nightmares.
The morning of Noctis' 5th birthday, Regis enters his son's room and sees Ardyn sitting on the bed, singing a lullaby with tears in his eyes.
Ardyn looks at him and whispers 'I'm sorry' because this sweet, gentle soul is going to die at his hand and it breaks his heart.