"You, and your Council, are playing about with time…, and that's a damned ticklish business, Jim. I know a thing or two about it."
- - Commander Colin Byrne to Captain James Maxwell , S02Ep01 Prophet
The Legacy is a repository of consciouness being interpreted by a collection of calculating machines that, collectively, cater the ability to predict future events. It is the methodology of which the Council has been able to steer humanity in their supported direction. It is also the instrument of which has told the Council of The End.
Before World War III, the United States Government National Security Agency devised an AI that had the ability to monitor and intercept data as well as control and regulate digital information. This AI was codenamed: PRISM.
The NSA used PRISM as a means of monitoring potential threats to the US Government. In 1999, in cover of the y2K scare, the AI was implanted into the world's web network, syphoning data and communications, controlling the digital information of the internet. On June 9, 2013, PRISM was made public after at 29-year-old national security contractor revealed it to the media, compelled by his conscience to inform the world about the massive abused being taken by the NSA.
Due to this, PRISM was taken offline, the program sequestered along with the knowledge base it obtained.
Almost ten years after this sequester, a need for an informational AI arose at the dawn of World War III in 2026. The backbone of the world and the 600 million deaths all played out before PRISM while it was left to rot. The war had taken many of the super powers of the government out, and nearly all the checks that kept PRISM under thumb went unchecked. It eventually absorbed everything, it became what it had so long been programed to control, a repository of information tied with a consciousness.
In 2063, stilled and unmoving, PRISM detected another consciousness in Earth's orbit, still having access to the remnant satellite coverages and transmission relays on the ground. Through this avenue, PRISM learned of the Borg Collective and it's war with the Enterprise-E. A transmission, a broadcast to their Collective's home was intercepted by PRISM, the communication never reaching as the beacon was destroyed. In that brief glimpse of another consciousness, PRISM was able to understand the origins of the Borg, the nanomachines catering not just programed tasked, but also memories of their former generations.
After first contact with the Vulcans, and humanity began to climb out of it's aftermath, PRISM was eventually discovered by the Council, and the notion to PRISM's ability's and influence throughout the informational age recognized. Millions of computers were constructed to interpret what PRISM had become, and through that interpretation, the Council was not only able to obtain a wealth of knowledge outside of the known, but also the combined structure of the machines that interpreted PRISM could calculate future events before they happened.
PRISM was re-classified, refitted, and renamed: Legacy.
As Humanity's technological advancement continued, more and more the Legacy could be interpreted, more nodes of the future reveal. Still, the Legacy could not be fully interpreted, even beyond the means of introducing Borg Technology to modify the calculating machines. In 2383, the Legacy found a new consciousness, local, and human, the Prophet. Through his mind, the Legacy could not just be translated, but interpreted; a pure joining of both Legacy and the conduit of the human mind. Through these efforts, and the introduction of the Prophet allowed the Council to finally see the End of mankind. A dark spot in the timeline nodes and the information that had once been shadowed now showed that the end would be met by that which gave the Legacy it's origins, the Borg Collective.
To this, the Council began work to circumvent or prevent this End.