Donnerstag aus Licht, performed at the Festival Hall on May 21 and 22

Donnerstag aus Licht is a crazy, cosmic epic but still somehow playful opera by Karlheinz Stockhausen about a musician/angel called Michael who floats celestially around the universe and has an abrasive encounter with Lucifer.

Older readers might remember the outing it had at the Royal Festival Hall in 1985 – another time, another world as it now seems, although it had sufficient impact for people to still talk about what happened.

And decades later it’s back there for two perfor­mances that if nothing else will be a nostalgia trip but may just summon up a sense of what the fuss was all about.

Stockhausen these days is an equivocal figure: a self-appointed seer of the mid-20th-century avant garde and more than slightly mad in that he claimed (with devastating seriousness as I recall from conversations with him) to have been born not here on Earth but on the star Sirius.

But extra-terrestrial ambitions aside, he was someone whose pioneer­ing work with electronic sound could be magical. He played a key role in the experimental music of his time. And he thought big. Which is how the already sprawl­ing Donnerstag (German for Thursday) came to be just one part of a massive, Wagnerian-style cycle of operas designed to follow the composer’s New Age narrative through all seven days of the week. A less than practical concept destined never to be realised.

This rare Southbank performance of the Thursday section features French ensemble Le Balcon, the London Sinfonietta, and a host of others. A must-see, must-hear event of the kind you tell your grand­children about.

• Performances on Tuesday and Wednesday May 21 and 22, from £15, 6.30pm, Royal Festival Hall, Southbank, SE1 8XX, 020 3879 9555, www.southbankcentre.co.uk