<P> When an old face returns to the castle and gains a position amongst the king's inner circle, Merlin must be on guard more than ever . For the new arrival is the sorcerer Mordred, the Druid boy whose destiny it is to end the King's life and bring chaos to Camelot . As the death song of King Uther haunts the castle and Guinevere crosses over to the dark side, both Merlin and Arthur find that their destinies are approaching . The battle for Camelot is about to reach a deadly conclusion and nothing will ever be the same again . </P> <Ul> <Li> Colin Morgan as Merlin </Li> <Li> Bradley James as Arthur Pendragon </Li> <Li> Katie McGrath as Morgana Pendragon </Li> <Li> Angel Coulby as Guinevere </Li> <Li> Richard Wilson as Gaius </Li> </Ul> <Li> Colin Morgan as Merlin </Li> <Li> Bradley James as Arthur Pendragon </Li>

When does gwen find out about merlin's magic