A depiction of Seb-Cheneb and her sister Seb-Natan on a pixmit card
Seb-Natan, also known as the Foreteller, was a character in the story of Ganmadan, or the "Day of Annihilation". Described as a khalagu ("demon"), she was the twin sister of Seb-Cheneb.
Seb-Natan played a drum made from the skin of children. She played it "so hard and so long" with a chain of skulls that her heart burst.
After the death of her sister, Seb-Cheneb blew the horn of the hellbeast Ganmadan, summoning the ch'khalagu ("very bad demons") who would bring about the Thousand Days of Pain, in which worlds were burned and lives were lost. (PIC: "The End is the Beginning", "Absolute Candor", "Et in Arcadia Ego, Part 2")
Michael Chabon elaborated on the myth of Seb-Natan in a Medium post. [1]