Arn: The Knight Templar (2010)

S1E6 The Kingdom at Road's End

Following a period of joy for Arn and his reunited family, King Knut's impending death triggers the threat of a brutal civil war. To avert a disastrous Danish intervention, the Folkungs orchestrate a plan where Karl, whose throne was usurped by Knut, is to succeed him but must accept Knut's eldest son, Erik, as crown prince. Sensing treachery, Arn discreetly sends his former Templar squire into Erik's camp as a supposed traitor, serving as Erik's protector. When he foils an attempt to drown Erik and his brothers—one of whom is tragically killed—Arn rallies for a revolt, preparing for a fierce battle.