A group of Zhat Vash at the Admonition in the 2380s
The Admonition was a preserved, encoded message on the planet Aia, placed some 200,000–300,000 years ago by the synthetic lifeforms who caused the extinction of organic life on that world. Documenting the development and evolution of the synthetics, and how organics caused their own destruction by seeking to destroy their creations, the Admonition served as a beacon to other synthetic lifeforms, offering protection against "oppression", even if it means the elimination of organic life. It contained an encoded frequency, through which they could signal the "alliance of synthetic life".
Data, as revealed to the Zhat Vash sisters, in a vision of the ancient memories stored on the device
Since the Admonition was designed for synthetic minds, any organics attempting to access it could be driven to madness, self-harm, and suicide. Hundreds of years before the 24th century, the Romulans discovered the Admonition while investigating the mystery of the Eightfold Stars. Mistakenly believing the Admonition to be a warning against synthetic life, the Romulans formed the Zhat Vash, a secret group dedicated to eliminating all synthetics everywhere. This effectively turned the Admonition into a self-fulfilling prophecy, as it caused the persecution it warned of; in turn, this persecution turned the legend of Ganmadan into a self-fulfilling prophecy by making future synths desperate enough to summon the "alliance".
In the 2380s, General Oh guided a group of Tal Shiar operatives, including Ramdha and Narissa, into becoming Zhat Vash by experiencing the Admonition. Some time later, a Borg cube assimilated Ramdha and suffered submatrix collapse due to the sheer weight of her despair.
In 2399, Oh passed the Admonition to Doctor Agnes Jurati via a mind meld, which subsequently drove Jurati to murder Bruce Maddox. Later, Sutra mind-melded with Jurati to gain the Admonition herself. (PIC: "Broken Pieces", "Stardust City Rag", "Et in Arcadia Ego, Part 1")