Okay. Well since you ended your history after you came out of the Deadlands, it leaves a lot open. At that point and beyond YC can never return to Myst. YC had a crisis of faith and maybe he ran into one of the Royal family in a different land. Perhaps they offered you a chance at a new life in Udir. *Bing* Debt. And the King would have been involved so Debt to the King.
Or
Maybe YC took up the fight against the Empire a few times. Perhaps one of those occasions YC was on the field of battle with the King. Perhaps the King recognized that you carried a sword of Myst and instead of killing you like everyone wanted he let you depart in peace.
Or
Perhaps you say YC's here because of a debt to the King, but maybe the King himself is in YC's debt? Maybe YC saved one of the royal family or even the King himself and because of YC's actions, the King knows he can trust YC.
Or
Something else. :D