His parents disappeared in the 40's, following a villain through a Timeslip into the future.
In their absence he became famous in his own right and represented the Authorities in the Nightside throughout the 60's and 70's. Hadleigh was the one who trained Walker, taught him everything he knew.
Something happened to Hadleigh then, although no one knows what. He left the Authorities and went underground, to the world beneath, into the sombre realms. There he found the Deep School. And, unlike many who never finish the course whole, he returned with all of his faculties. Hadleigh Oblivion, The Detective Inspectre, who only ever investigates crimes and cases where reality itself is threatened.
As a graduate of the Deep School, Hadleigh Oblivion is said to be enormously powerful and something more than human.
He has healed and destroyed, killed and saved countless people. It is said that Hadleigh prevented the Angels from Above and Below from interfering in the Lilith War. He knows what he needs to know when he needs to know it.