Board Thread:Spoilers!/@comment-11468987-20151019022445/@comment-6175354-20151019023053

According to Merlin's prophecy, Camelot's rightful heir is known as the Orphan King. It seems that Arthur misinterpreted that it meant him. While he does fit the description as he is an orphan, the prophecy is referring to someone else. It would appear that towards the end of the episode, OUaT forshadowed who the true Orphan King is.