A depiction of Seb-Cheneb and her sister Seb-Natan on a pixmit card
Seb-Cheneb, also known as the Destroyer, 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-Natan. Seb-Cheneb was said by Romulans to be the "end of all".
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")
In 2399, the Romulan xB Ramdha accused Soji Asha of being Seb-Cheneb, after seeing the sisters depicted on the final pixmit card in her hand. Ramdha subsequently became hysterical and pulled a disruptor on Asha before attempting suicide.
The Zhat Vash also referred to Asha as Seb-Cheneb. (PIC: "The End is the Beginning", "Absolute Candor")
Michael Chabon elaborated on the myth of Seb-Cheneb in a Medium post. [1]