Spoiler warning: Plot and/or ending details follow.
- "What is Zhat Vash?"
"A Tal Shiar myth. It's a kind of boogeyman they use to frighten new recruits, and children."
- —Jean-Luc Picard & Zhaban, 2399[src]
The Zhat Vash was a ancient Romulan cabal that predated the Tal Shiar. Zhat Vash was also a term used to refer to the dead who were the "only reliable keepers of secrets". (PIC episode: "Maps & Legends")
Hundreds of years before the 24th century, Romulans discovered the world of Aia at the center of an octonary star system they named the Eightfold Stars. There, they found an artifact known as the Admonition. The artifact showed memories of devastation carried out by synthetic "Destroyers," some 200,000 to 300,000 years earlier. The discovers of the artifact dedicated themselves to eliminating synthetic lifeforms to avoid this fate, and became the first Zhat Vash. They recruited followers to their cause by showing them the Admonition in order to strengthen their belief in their cause. (PIC episode: "Broken Pieces")
By the 24th century, The Zhat Vash were considered just a boogeyman story told to frighten Tal Shiar recruits such as Zhaban, his mother, and his wife, Laris. The Zhat Vash operated across the galaxy without regard of treaty or jurisdiction. Their theater of operations went beyond Romulan worlds to places such as the Klingon Empire, the Gorn Hegemony, and the United Federation of Planets. (PIC episode: "Maps & Legends")
In 2385, one of the Zhat Vash's deep cover operatives, Starfleet Security Director, Commodore Oh recruited several more operatives from Tal Shiar. Only 2, Narissa and her aunt Ramdha survived the admonition. The Zhat Vash were suspected in planning the attack on mars, which forced the Federation to ban synthetic life-forms and assisting the Romulan Empire's evacuation. (PIC episodes: "Broken Pieces", "The End is the Beginning", "Et in Arcadia Ego")
In 2399, several members went to Greater Boston. There, they attempted to capture Dahj Asha for information on other androids and their location. However, she was able to kill them all when her programming kicked in. Several operatives continued their attempt to capture Asha and eliminate retired Admiral Jean-Luc Picard. Asha was able to kill them, but one was able spit acid at her and on her disruptor which destroyed her. They were able to cover up their tracks from Picard by erasing themselves and Asha from Starfleet Headquarters' computers. They were also able to remove their traces from Asha's apartment with antileptons.
Commodore Oh was informed of Picard's plans when Starfleet CNC, Fleet Admiral Kirsten Clancy asked her if she was aware of Commander Bruce Maddox's whereabouts and if there were any covert Romulan operations on Earth. After lying to the Clancy, Oh brought in "Lieutenant" "Narissa Rizzo". Oh told Rizzo about Picard's knowledge of the Zhat Vash and Maddox. Rizzo told her that she could take care of Picard and had one of her best agents tracking Soji. Rizzo's agent, her brother Narek had found Soji at the Artifact. (PIC episodes: "Remembrance", "Maps & Legends")
Oh later met up with Dr. Agnes Jurati. There, she mind melded with Jurati and showed her the Admonition's message about the threat of synthetic lifeforms. Several more members attacked Picard at Château Picard after disabling the alarm. Laris, Zhaban, and Jurati were able to take out and capture one of Zhat Vash members. (PIC episode: "The End is the Beginning")