The Zhat Vash was the name of a secretive Romulan cabal that was introduced in the Star Trek: Picard pilot Remembrance.
Some time before the 24th century, Romulan explorers came across a star system consisting of eight stars with a single habitable planet in the middle. On that planet, called Aia, they found a device they would call the Admonition. The memories shown to them by the Admoniotion of synthetic life killing the race created the Admonition so horrified the Romulans that they became determined to destroy all synthetic life throughout the galaxy. Romulan society became imbued with a pathological hatred of artificial intelligence and artificial life. The original group would become the Zhat Vash.
A highly secretive group the Zhat Vash operated without regard for treaties not only within the Romulan Empire, but also in Klingon and Gorn space as well. They were so secretive that many Romulans did not even know of them, and even some members of the Tal-Shiar dismissed them as rumors.
The group kept the Admonition secret, subjecting new recruits to the device to test their worthiness to join the Zhat Vash. Experiencing the Admonition was not an easy task, with many recruits driven to madness and suicide.
As the Zhat Vash became aware of Dr. Noonien Soong's work to create androids with positronic brains that would in many ways be superior to biological life, they sent one of their operatives, a young female Romulan/Vulcan hybrid named Oh, to infiltrate Starfleet and work to stop the rise of synthetic life.
Over the years Oh rose to the rank of Commodore and became the head of Starfleet security, where she used the resources of the Federation to further their mission to destroy all synthetic life. Learning that Doctor Bruce Maddox had worked to create new androids from positronic neurons coming from the late Commander Data, Oh and her agents set out to destroy these synthetic lifeforms at any costs.
In the 2380s Oh had Narissa Rizzo and Rizzo's aunt Rhamda participate in the Admonition. All the other recruits were driven to suicide while Rhamda was driven insane. Only Rizzo managed to hang on to her sanity.
Oh worked with Rizzo to turn the synths working on Mars rogue. The synths launched a devastating attack on Mars which cost the lives of over 90,000 people and led to a ban on synthetic lifeforms in the Federation.