One of the twin demon sisters of the Romulan myth of Ganmadan, the Destroyer who sounds a horn to unleash annihilation, and whom Narek comes to identify with the synthetic Soji Asha.
Seb-Cheneb is a figure in the Romulan story of Ganmadan, the day of annihilation. The tale speaks of twin sisters, both demons. Seb-Natan, the Foreteller, beats a drum made from the skin of children until her heart bursts; Seb-Cheneb, the Destroyer, carries a horn taken from a great pale hell-beast and blows it to release all the demons who have waited since the beginning of time.
Narek recounts the legend in the Picard first-season finale, Et in Arcadia Ego, Part 2, beside a campfire near the grounded La Sirena. He explains that the Zhat Vash treat the story not as myth but as prophecy, a memory of the Admonition's warning about synthetic life, and that he believes Soji to be Seb-Cheneb, about to summon an alliance of machine intelligences.
Neither sister is depicted; the name gives shape to the Zhat Vash's centuries of fear.