The Kasseelians were an Alpha or Beta Quadrant species. They were known for the Kasseelian opera genre of music.
Female Kasseelian prima donnas trained their whole lives for a single performance, after which they typically committed suicide by plunging a dagger into their own chest. (DIS: "Magic to Make the Sanest Man Go Mad", "Brother")
A Kasseelian opera house was present on a moon near Starbase 46 as of December 2256. (DIS: "Into the Forest I Go")
This species was only mentioned in dialogue..
A Kasseelian design was developed by Neville Page for a Star Trek: Discovery scene to have featured Paul Stamets watching a memory with Hugh Culber. Though the scene was changed, the design was instead used as the being in ST: "The Girl Who Made the Stars". (Star Trek: The Art of Neville Page, p. 106)