Hagen is a Burgundian and chief vassal (landowner giving military aid to the king) of the Burgundian king. His title is Lord of Troneck. Hagen was captured at one time by the Huns (Hungarians) and held hostage at Etzel of Hungary's court. Etzel of Hungary is also known in history as Attila the Hun.
Hagen is responsible for having killed Siegfried who was lord of Netherlands, Norway, and Nieblungland and husband of Kriemhild, who was princess of Burgundy before her marriage. After Siegfried's death at Hagen's hand, Kriemhild married Etzel and so is the wife of the king who held Hagen hostage.
Rudiger on the other hand is a Hun and the vassal of Etzel, so he was one of Hagen's captives while he was held hostage. It is Rudiger who travels to Burgundy to present's Etzel's offer of marriage to Kriemhild after Siegfried's death. As a vassal to the king of Hungary, Rudiger swears an oath of loyalty to Kriemhild when she becomes queen and is therefore bound to aid her in her revenge plot against Hagen for killing Siegfried.
Rudiger's position is complicated to the level of tragedy because he marries his daughter, a Hun, to Giselher, a Burgundian and Kriemhild's brother. Kriemhild's revenge plot against Hagen is not deterred because she is Burgundian and Hagen, Siegfried's murder, is also Burgundian. According to Rudiger's oath of loyalty, Rudiger is the enemy of Hagen the Burgundian, a relationship which calls for Rudiger to wage war against Burgundy, his daughter's new land.