The Wagnerian sized Gurre-Lieder by the Austrian composer Arnold Schönberg is a collection of songs, chorusses, instrumental interludes and speech-singing in the form of an oratorio followed by an epilogue for five solo voices. If you like Wagner, this is the next thing. In a way this is what Wagner would have done if he would have lived, say, fifty years later.
The story is one of damnation and redemption. King Valdemar of Denmark (a twelfth century historical king), residing in Gurre castle, fell in love with his mistress, Tove. When his wife slaughters her, Valdemar curses God. Because of that, he is cursed after his death to ride the skies looking for his beloved Tove, for all eternity. Of course he never finds her.
That basic story is told in three parts. The first part consists of love poems between Valdemar and Tove. The second describes the cursing of God, and the third describes Valdemar scaring a peasant during his nightly hunt for Tove.
The work grew out of a few songs that Schönberg wrote for a competition in 1900. He then kept working on it, making it bigger and bigger until 1903. The there is a hiatus of seven years, in which Schönberg composed some quite successful works, and by 1910 his musical world was totally changed. He was greatly influenced by Mahler and by then his twelve tone theory had more fully developed. Changes in orchestration were a consequence, but they do fit the drama. It does mean that listening the complete work kind of takes you along on this musical development.