Kasseelian opera was a genre of music. (DIS: "Magic to Make the Sanest Man Go Mad", et al.) It featured musical instruments and singing performed by Kasseelian prima donnas. Such performers trained their whole life for a single performance. After she hit the last high E note, the Kasseelian prima donna typically committed suicide by plunging a dagger into her own chest. (DIS: "Brother")
When Paul Stamets and Hugh Culber met for the first time, Culber was humming Kasseelian opera. Stamets found this atrocious and told him to "stifle it or sit somewhere else." Culber came and sat next to him instead. (DIS: "Magic to Make the Sanest Man Go Mad")
There was a Kasseelian opera house on a moon near Starbase 46. In December 2256, Stamets invited Culber to see a showing of La bohème there. (DIS: "Into the Forest I Go")
After Stamets became trapped inside the mycelial network, he interacted with a manifestation of Culber within a replica of their quarters. He asked the computer to play a Kasseelian opera aria that he hated but Culber loved. (DIS: "Vaulting Ambition") Later, the music guided Stamets as he attempted to navigate the USS Discovery from the mirror universe back to their original universe. (DIS: "What's Past Is Prologue")
As Culber predicted, Stamets later came to an understanding of why Culber had been fond of Kasseelian opera, believing this was due to "the tonal matrices woven together, the weird rapture between instruments and voice." (DIS: "Brother")
Doctor Agnes Jurati was listening to Kasseelian opera outside the Daystrom Institute in 2399 when she was visited by Commodore Oh. (PIC: "The End is the Beginning", "Nepenthe")
Kasseelian opera is audible in the soundtracks for "Vaulting Ambition", "What's Past Is Prologue", and "Brother".
The Kasseelian opera was scored by Composer Jeff Russo, who found it enjoyable to do so. "That was really super fun to do and super amazing," he enthused. [1]