FRIENDS gather of an evening to witness the once-in-nine-yearly flowering of an exotic cactus. While they’re waiting for this miraculous night bloom, they keep themselves entertained in time-honoured tradition of telling heroic tales and reciting epic poems. The last recitation is the Gurresange – a grand tragedy of illicit love and jealous fury and the transcendental powers of nature to resurrect the human spirit.

It goes something like this. Back in 14th century Denmark, King Waldemar loves the beautiful maiden Tove and they meet for secret passionate trysts at the castle of Gurre. Queen Helwig poisons Tove in a jealous rage (the original ballad has her locked in a sauna, which seems a uniquely Scandinavian revenge) and Tove’s funeral is recounted in vivid verse by a wood dove. The grieving Waldemar curses God and is condemned to forever fly through the night skies, while Tove is splendidly transfigured through the glories of nature and, to top it all off, the poet himself appears to reassure us of the renewing forces of each new sunrise.

All sounds a bit Wagnerian? Well it isn’t: this is Arnold Schoenberg – or rather, this is Jens Peter Jacobsen’s 1868 novella A Cactus Blooms on which Schoenberg based his humungous choral-orchestral cantata Gurrelieder, which is the closing work of the 2016 Edinburgh International Festival on Sunday. Schoenberg’s lasting legacy would be as the father of atonalism, kick-starter of 12-tone modernism, rather humourless inventor of an iron-clad music theory that prioritised concision and logic and rejected precisely the overheated chromaticism and subjective expressivity of late Germanic romanticism – and of Gurrelieder. For some listeners, Schoenberg is still the bogeyman and the reason everything went so wrong in 20th century music.

But even Schoenberg was young once, and at 26 he had just finished writing his ecstatically quivering tone poem Verklärte Nacht (Transfigured Night) based on a poem by Richard Dehmel that also dealt in such romantic favourites as nature and nighttime. In 1899 he set to work on a German translation of Gurrelieder and by 1901 had finished a short score that it would then take him ten years to fully flesh out. Partly that’s because the forces are so gargantuan: an orchestra of 140 including seven trombones, seven clarinets and eight flutes, plus a mixed choir, three male choruses, five vocal soloists and a speaker. It’s no wonder Schoenberg took his time wrestling with the sheer mechanics of the thing.

But he was also busy revolutionising western classical music as anyone knew it. By the time Gurrelieder finally premiered in 1913 he had already written paradigm-changing atonal masterpieces – Pierrot Lunaire, Erwartung, the Second String Quartet, the Five Orchestral Pieces. So the eventual arrival of Gurrelieder felt like a time-lapse: he had moved on but audiences in Vienna had not. The irony was that Gurrieder’s thickly outmoded romanticism was instantly more popular than any of Schoenberg’s avant-garde works would ever be.

Schoenberg was sceptical about the positive reception. “Unfortunately, concerts in Vienna are not set up as artistic affairs,” he told a correspondent of Die Zeit in 1913. “They are political ones. How a thing should be received is determined in advance; people come to a concert with their opinions already firmly in place. In my view, that undermines the success of the Gurrelieder.” He would never return to that world of sunlit dawn choruses and ecstatic love duets.

So how should we listen to Gurrelieder today? For mezzo-soprano Karen Cargill – who sings the Wood Dove at the Usher Hall on Sunday – all those issues around authorship and Schoenberg’s modernist trajectory pale into irrelevance as soon as the music starts.

“I absolutely love this piece,” she says. “If you were to put it on, nobody would ever guess it’s Schoenberg with his scary modernist reputation. Wagner, maybe, or Mahler, both of whom influenced Schoenberg massively.” She says the writing for the mezzo voice is masterful – "he does these wonderful floaty Mahlerian things that allow us to find absolutely gorgeous silvery colours” – and she marvels at the rich textures, the “dream-like poetry” that “unfolds in wonderfully clear storytelling."

"His characters are beautifully written. The love duets between Tove and Waldemar are sublime, reminiscent of Act Two of Wagner’s Tristan und Isolde. It’s almost operatic. And my big Wood Dove aria is breathtaking, the high dramatic point of the whole piece. Though,” she chuckles, “I say that! Basically, anyone who likes big Germanic romantic music with big tunes and bigger emotions – this is for you.”

Talking of big emotions, Sunday night is Donald Runnicles’s last concert as chief conductor of the BBC Scottish Symphony Orchestra. It’s the job that brought the Edinburgh-born conductor back to Scotland seven years ago and the relationship that has made the BBCSSO sound superbly powerful and burnished in Germanic romantic repertoire. Alongside Cargill, Sunday’s lineup of soloists includes soprano Anja Kampe, tenor Simon O’Neill, bass-baritone Iain Paterson and Thomas Quasthoff as speaker. It is a line-up typical of the heavyweight Wagnerian voices Runnicles has worked with around the world and brought back to Scotland.

For a singer like Cargill, the departure of true singers’ conductor like Runnicles is a sadness. They first worked together nine years ago and “he now knows me so well,” she says. “He knows before we even start rehearsals what kind of colours I will want to make. He’s wonderful that way with singers – he has the perfect sense of where to give us space to breathe, how to cushion us with the most beautiful orchestral sounds. He lives and breathes this Germanic romantic music and has shared that expertise with Scotland. What a gift!”

Runnicles ends his post with the BBCSSO on the very same stage that began his career as a 12-year-old boy soprano singing in the inaugural Edinburgh Festival Chorus. (The work that night in 1965 was another romantic mega score: Mahler’s Eighth Symphony, for which the chorus master Arthur Oldham had recruited 240 adults and 100 boys.) “The thing about Donald,” says Cargill, “is that even though he has conducted around the world at the greatest opera houses, you still see the spark in his eye about what it means to be back at the Usher Hall. To come back to Edinburgh, to conduct a piece like Gurrelieder on his home turf… This concert will be an extraordinary celebration of what he and the BBCSSO have accomplished together.”

Karen Cargill sings Gurrelieder with Donald Runnicles conducting the BBC Scottish Symphony Orchestra the Edinburgh Festival Chorus at the Usher Hall on Sunday.