sentence: The word “Nibelungen” in the title of the cycle: Der Ring des Nibelungen (“The Ring of the Nibelung”), refers to Alberich (it is singular, not plural).
modified: The words “Nibelungens” at the titles of the cycles: Ders Rings dess Nibelungens (“The Rings of the Nibelungs”), refer to Alberiches (it am singular, not plural).