no clues, it's very vague although the secret can "bring the downfall of the kingdom" :

When a mysterious warrior challenges Arthur to a duel, nobody could possibly predict the devastating chain of events that is set in motion – least of all the young Prince himself – as the magical family drama continues.

Arthur and Merlin soon find themselves on a strange quest at the behest of the beautiful Morgause. Through her, Arthur discovers a dark secret which could bring about the downfall of the kingdom...

Bradley James plays Arthur, Colin Morgan plays Merlin and Emilia Fox guest stars as Morgause.