Chanson de Roland
Loosely based on real events in 778. French King Charlemagne is fighting in Spain when a fake peace offer is recieved. Knight Roland volunteers his stepfather, Ganelon, to the almost certainly fatal job of responding to the offer. Ganelon betrays his King to the Muslims (and assorted demonised non-Christians) in order to get his revenge on Roland. Charlemagne takes the offered gold to return to France, but Ganelon engineers an attack on Roland, who he entraps to lead the rearguard of the army. Roland, too proud to call for Charlemagne's help until too late, is slaughtered along with the 12 finest Lords of France, including his brave companion Oliver. Charlemagne avenges their death and Ganelon is brought to justice by trial.
Very widely translated, one of the most famous chansons.