Who is the dead king, and what exactly is the purpose of the place other than the treasure?
The King’s Grave
I think it's just an interesting little tidbit, but I'm not sure on that.
Perhaps it has to do with
what happens at the end of the game - (see double spoiler if you really want to know
Double spoiler: MAY RUIN END GAME EXPERIENCE. YOU HAVE BEEN WARNED.
... it says "To Be Continued". Perhaps the King was somebody important that the son will run into.