This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
"A Tal Shiar myth. It's a kind of boogeyman they use to frighten new recruits, and children."
The Zhat Vash was an ancient and secret Romulan cabal of Tal Shiar operatives. According to Laris, it was thousands upon thousands of years old, and it supposedly predated the Tal Shiar, which she said functioned merely as a mask for the Zhat Vash. Zhat vash was also a term sometimes used to refer to the dead, the "only reliable keepers of secrets". (PIC: "Maps and Legends")
Its sole purpose was claimed to be to keep a secret so profound and terrible that just learning it can break a person's mind. When she was a Tal Shiar recruit, Laris heard a drunk handler claim to her that the organization was motivated by hate, fear and pure unassuageable loathing for artificial intelligence, including androids, AIs, and other forms of synthetic life, which they attempted to seek out and destroy. (PIC: "Maps and Legends")
Allegedly, the Zhat Vash operated without regard for treaty or jurisdiction, not just on Romulan worlds, but also in the Klingon Empire, the Gorn Hegemony, and even the United Federation of Planets. (PIC: "Maps and Legends")
The Zhat Vash were formed hundreds of years before the 24th century, when a group of Romulans investigating the mystery of the Eightfold Stars discovered the Admonition on the planet Aia. After experiencing the memories of the utter devastation wrought by the synthetic "Destroyer", called Seb-Cheneb, contained within the Admonition, they dedicated themselves to eliminating synthetics so as to prevent another such event. Subsequent generations of Zhat Vash would also submit themselves to Admonition, so as to strengthen themselves for their task. (PIC: "Broken Pieces")
The story of the "Zhat Vash" was sometimes told to frighten Tal Shiar recruits, with its alleged motives known to very few; Zhaban was largely skeptical of their existence and equated it to stories of "boogeymen". In 2399, former operative Laris related the rumors about the Zhat Vash to retired Admiral Jean-Luc Picard; they came to believe that the Zhat Vash were likely behind the attacks on Dahj Asha on Earth. (PIC: "Maps and Legends")
General Nedar later led a warbird squadron to destroy the planet's population of synths, but encountered orchids and a fleet of Starfleet starships led by the USS Zheng He. During that battle, Narissa was killed in combat with Seven of Nine on the Artifact. It was that event that the Federation came to believe that the Zhat Vash were likely behind the Attack on Mars and lifted the ban on creation of artificial lifeforms. (PIC: "Et in Arcadia Ego, Part 1", "Et in Arcadia Ego, Part 2")