The alliance of synthetic life was an extra-galactic and possibly trans-dimensional organization composed of synthetic lifeforms. Little else was known about them other than they were fiercely hostile to organic life and were willing to "free" any group of synthetics from being enslaved by "organics" whenever they would receive a signal from said synthetics.
In 2399, synthetics created by Altan Inigo Soong on the planet Coppelius attempted to make contact with the alliance from Coppelius Station, using a beacon specially constructed for that purpose. Encoded within the message of the Admonition was information concerning how to contact this synthetic civilization, but this information was only detectable by synthetic life forms who were exposed to the Admonition's visions.
Amid the defense of Coppelius, Jean-Luc Picard was able to convince Soji Asha to shut down the beacon by offering to give his life in exchange for protecting them against the Zhat Vash. (PIC: "Et in Arcadia Ego, Part 1", "Et in Arcadia Ego, Part 2")