The Kingdom at Road's End - S1.E6

After happy years for Arn and his finally reunited family, king Knut is dying, spelling a bloody civil war. Fearing a Danish intervention would be fatal, the Folkungs arrange a deal by which Karl, whose father's throne Knut re-usurped, succeeds but must adopt Knut's firstborn Erik as crown prince. Rightly suspecting foul play, Arn sends his Danish former Templars squire along as supposed turncoat, in fact Eric's bodyguard. After he prevents Eric and his brothers, one of which is however slain, being cowardly drowned, Arn takes command of a revolt, heading for an epically bloody battle.