Merlin - Season 5 Episode 8

Earnest young druid Daegal begs Merlin to come to his village and save his dying sister, but the boy leads him into a trap where he's left to die by Morgana. Arthur meanwhile hosts the ruthless Sarrum of Amata, with whom he plans to form an alliance; however, with Merlin deliberately put out of the way, the bewitched Guinevere, appealing to the Sarrum's lust for power, persuades him to assassinate Arthur on the promise of a share of the kingdom. He accepts (though plans on killing her, too). Guilt-ridden, Daegal returns to save Merlin and they make it back to Camelot in time to thwart the assassination, though with several casualties. Merlin is more determined than ever to discover the cause of Guinevere's unnatural actions.