This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
Ganmadan was what ancestral Romulans called the Day of Annihilation, an event in which all life everywhere would be destroyed. The story of Ganmadan shared a common theme with the Human stories of Ragnarok and the Judgment Day and, like those stories, was an ancient myth. However, some Romulans, including Narek, believed the story was not a prophecy. He believed it was history and that it could happen again.
The story began in the distant past, with some Romulans believing it occurred before their ancestors arrived at Vulcan, with a pair of khalagu, the twin sisters Seb-Cheneb and Seb-Natan. After the death of Seb-Natan from a burst heart while playing her drum, Seb-Cheneb blew the horn of a pale Ganmadan, summoning the ch'khalagus who brought about the Thousand Days of Pain.
In 2399, Doctor Soji Asha watched a recording of Professor Ramdha being asked about Ganmadan on the talk show Yrrh Mnrrh. (PIC: "Absolute Candor") Later that year, while sitting around a campfire on Coppelius, Narek narrated a abbreviated version of the story to Cristóbal Rios, Raffaela Musiker, and Elnor. (PIC: "Et in Arcadia Ego, Part 2")