The Legacy Bearers were living beings, once, but that was a very, very long time ago.
2.3 billion years ago, a starship landed on primordial Earth. It was an Ark, bearing the last survivors of a race of aquatic cephalopods whose homeworld had been destroyed when its sun went nova, and its mission was nothing less than to recreate that lost world on Earth.
The cephalopods' native planet had somewhat resembled Earth during the Jurassic period - hot and oxygen-rich. Earth, with its sulfurous atmosphere and near-total lack of life, was nothing like it, but it was the closest match they'd found in almost three centuries of searching, and the Ark was reaching the limits of its operational life.
And so the cephalopods, the ancestors of the beings who would one day become the Legacy Bearers, set to work to transform their new home. To increase the amount of oxygen in the atmosphere, they released vast quantities of cyanobacteria, causing what modern science now knows as the Great Oxygenation Event or Oxygen Catastrophe.
But the ur-Legacy Bearers miscalculated the climactic effects of such a massive release of oxygen. The Huronian glaciation caught them completely by surprise, and despite their best efforts, they found themselves unable to undo the multi-million-year global ice age which they had unleashed. They cannibalized their ship to construct great undersea bunkers, and retreated into them to try to wait out the cold.
A million years into the eternal winter, the species had changed dramatically. Their grasp of physical science and technology had gradually slipped away, even as their understanding of psionics, abstract mathematics, and ultimately, Block Transfer Computation, steadily advanced. But the relentless attrition caused by the harsh climate was shrinking their gene pool, and they realized that it was only a matter of time before they faced extinction.
Their solution was to transcend the limitations of the flesh and to some extent, the limitations of time itself. Combining their psychic abilities and mathematical knowledge, they transformed themselves into elemental beings, living matrices of Block Transfer Computations existing timelessly in a parallel dimension, able, with equal ease, to reach into the past to recover all the knowledge that they had lost, and the future, to remake the world.
Or that, at least, was the theory. But the Legacy Bearers had once again miscalculated. They could see through time easily enough; as they had planned, their transformation completely restored their understanding of their species' history, culture and technology. But they were trapped in the alternate dimension that they now inhabited, coterminous with the physical world but unable to influence it in any way.
The emergence of intelligent life offered them a chance to escape their prison. They learned that it was possible to communicate with psychically gifted beings across the dimensional barrier, and even influence their actions. A sufficient number of minions, working together, would theoretically be able to open the dimensional barrier from the other side. The Legacy Bearers' first cult, among the three-eyed terrestrial Silurians, was forced into hibernation along with the rest of their species, and it took millions of years for a new species - humanity - to develop the same potential.
The Legacy Bearers want to remake Earth as a duplicate of their former home world, including re-creating the sapient species that they themselves once belonged to. To achieve this end, all extant terrestrial life must be extinguished. Before they transformed themselves, the enormity of that idea might have given them pause, but their "elevation" to a higher state of being has subtly warped them. In effect, the Legacy Bearers no longer have free will; each is a embodied mathematical construct, more akin to a sentient computer program than a living being. They pursue their goal because that's what they programmed themselves to do when they altered themselves; they don't feel a need to justify their actions and they're no longer even capable of questioning their own motives. They are, however, more than capable of deceit and deviousness in pursuit of their ultimate goal; the cultists who they lure into serving them are (in most cases), blissfully unaware of their masters' real objective.