Der Ring des Nibelungen
Wagner spent about twenty years writing the Ring cycle.  He started in the 1850s when he was living in Switzerland.  He wrote all the libretto (the words) himself, as he always did for his operas.  By 1857 he had finished the first two operas and two thirds of Siegfried.  It was many years later that he finally finished Siegfried and wrote the final opera as well.