In both the light novels and the Animé, the Andvari ring was an important tool for Louise and Saito's opponents. He was able to make people mindless and to make them influenced. He was Sheffield's main tool in facilitating the formation of the Reconquista of Albion and causing serious damage to the combined armies of Tristain and Germania.
In the Animé, the ring was used to poison the drinking water of the city of Gotha and to turn all who drank from it into mindless zombies whose main goal after the influence of Sheffield was to kill Queen Henrietta.
Originally the ring had belonged to the water spirit of Lake Lagedoria, but King Joseph and his Sheffield familar had managed to steal it. In order to regain it, the water spirit had flooded the shores of the lake. Saito promised the water spirit to get him back the ring, which happened in the light novels in the last chapter.
Trivia: In the Siegfried saga, the ring was the main piece of the Nibelung's treasure and originally belonged to the dwarf Andvari. The god Loki had caught Andvari and pressed the whole treasure from him for his release, whereupon Andvari put the whole treasure and especially the ring with a curse. From then on, it was supposed to bring misfortune to every owner of the treasure, which later ruined the hero Siegfried.