Tragedy strikes in Arthur's court as this week's Merlin begins - a sorcerer named Osgar (played by solid character actor Andrew Tiernan) has killed one of the King's knights, Sir Ranulph. We've never seen or heard from Sir Ranulph before, but are assured he was a terribly nice chap - think of him as Sir Not-Appearing-In-This-Film from Monty Python and the Holy Grail. When the remaining knights eventually corner Ranulph's killer, Gwaine (Eoin Macken) is quick to stick the boot - or rather the sword - in, but Osgar survives long enough to deliver a message from creepy soothsayer trio the Disir, who have sworn vengeance on Arthur for turning his back on the old religion. Arthur (Bradley James) receives this chilling omen with Mordred (Alexander Vlahos) at his side, the newcomer having been promoted to active knight duty after impressing the king in a training bout. Of course, we all know that Mordred's apparently fated to kill his liege, but perhaps even more terrifying for fans of this show is the possibility that he presents a threat to the Merlin / Arthur bromance - we can imagine 'Merthur' obsessives quaking in their boots as the king waxed lyrical about his new knight's swordplay. The subject of much good-natured jostling from his fellow knights, Vlahos's wide-eyed warrior is easy to sympathise with. Merlin (Colin Morgan) speaks for the viewer when he admits that he can't help but like Mordred, despite what we all know about his ultimate role in Albion's downfall. After his complete absence in the previous episode, Mordred's relationship with Merlin continues to evolve in interesting ways this week, with our hero doing his level best to reign in his fellow sorcerer's dark side. The scene at Osgar's grave is a particular stand-out, as Mordred stares through Merlin with dark eyes one moment and flashes him a smile the next - he's a truly unpredictable figure. Back in Arthur's court, Gaius (Richard Wilson) may not buy into Merlin's fears about Mordred, but he's very concerned indeed about the Disir's warning for Arthur - in particular the rune-mark predicting his death... We know throughout 'The Disir' that Arthur is unlikely to reverse his stance on magic - such a massive change in the show's status quo would be oddly placed at this mid-series juncture. Still, it's interesting to watch the young monarch's sense of moral righteousness waver as he pays heed to the Disir's warnings and weighs up his past 'transgressions' against the old religion. Merlin too has the Disir's warning weighing heavily on his mind and is faced with a dark choice when the Great Dragon implies that he should kill Mordred to save Albion. When a determined Arthur decides to ride out and face his accusers, Mordred again accompanies his king - but rather than presenting a threat to Arthur's life, he ends up sacrificing himself to save it... Badly wounded in a brutal attack, Mordred is seemingly on the way out and while Arthur is desperate to save his "brother-in-arms", Merlin - the only man with the power to save him - is weighing up the option of letting his future nemesis die. Colin Morgan plays the emotional crisis with skill and Gaius calling his ward on his increasingly dark nature is a highlight of the episode. When Arthur ultimately decides to return to the Disir and beg for mercy, he's met with an ultimatum - agree to embrace magic by dawn or Mordred will die for certain. Now Merlin's faced with an even larger dilemma - if Mordred lives, Arthur may die, but if Mordred dies, the dream of a Camelot where magic is no longer outlawed could die with him. There's more excellent work to enjoy from Colin Morgan here - the Irish star looks fit to burst with emotion when his friend considers, "Perhaps the old ways aren't as evil as we thought..." Though Arthur eventually decides to spurn the Disir's offer, Mordred makes a miraculous recovery and, in a final twist, it transpires that Mordred's survival is in fact Arthur's punishment for turning his back on magic once again - is he fated to kill the King after all? 'The Disir' is another engaging episode in what is proving to be a strong fifth run for Merlin. Last week's 'Another's Sorrow' admittedly saw our interest dip a little, but this Saturday, the show is back to its best.