When was the last time the Rhinemaidens were seen in the four opera cycle?
The Rhinemaidens are the three water-nymphs (Rheintöchter or "Rhine daughters") who appear in Richard Wagner's opera cycle Der Ring des Nibelungen. Their individual names are Woglinde, Wellgunde and Flosshilde (Floßhilde), although they are generally treated as a single entity and they act together accordingly. Of the 34 characters in the Ring cycle, they are the only ones who did not originate in the Old Norse Eddas. Wagner created his Rhinemaidens from other  legends and myths, most notably the Nibelungenlied which contains stories involving water-sprites (nixies) or mermaids. The key concepts associated with the Rhinemaidens in the Ring operas—their flawed guardianship of the Rhine gold, and the condition (the renunciation of love) through which the gold could be stolen from them and then transformed into a means of obtaining world power—are wholly Wagner's own invention, and are the elements that initiate and propel the entire drama.
The Rhinemaidens are the first and the last characters seen in the four-opera cycle, appearing both in the opening scene of Das Rheingold, and in the final climactic spectacle of Götterdämmerung, when they rise from the Rhine waters to reclaim the ring from Brünnhilde's ashes.  They have been described as morally innocent, yet they display a range of sophisticated emotions, including some that are far from guileless. Seductive and elusive, they have no relationship to any of the other characters, and no indication is given as to how they came into existence, beyond occasional references to an unspecified "father".
The various musical themes associated with the Rhinemaidens are regarded as among the most lyrical in the entire Ring cycle, bringing to it rare instances of comparative relaxation and charm. The music contains important melodies and phrases which are reprised and developed elsewhere in the operas to characterise other individuals and circumstances, and to relate plot developments to the source of the narrative. It is reported that Wagner played the Rhinemaidens' lament at the piano, on the night before he died in Venice, in 1883.
final climactic spectacle of Götterdämmerung