Forever.
When the first of the four-legged creatures wandered into the ruins, years after the last of the creators had vanished, Antrax was ready. It had probed its memory banks for the details of the defenses and weapons that had been given to it, and it used them. Lasers effortlessly cut apart many of the intruders. Metal sentries and fighting units chased down the rest. The four-legged creatures were no challenge, but they gave Antrax a chance to test its ability to fulfill its directive.
Later, humans tried to venture into the ruins, as well, to explore the collapsed chambers and crumbling passageways, even to find their way underground. None of them had the code. Antrax destroyed them all. Yet others returned from time to time, some of them becoming recognizable by their look and feel, some of them persistent in their efforts. Like ants, they tunneled and burrowed, little annoyances that refused to be chased away for more than a short time. Even the lasers and probes failed to discourage them.
Antrax began to explore other solutions. It found interesting possibilities in its memory banks and experimented with them. The wronks proved the most successful. Something about revisiting the dead was especially frightening to humans.
They gave it a name. Antrax. They took it from their own language. Antrax had no idea what it meant. Nor did it care to know. What mattered was that they knew it was there. That was enough to accomplish what was needed. The humans began to avoid the ruins. They no longer spent time searching for entrances into the catacombs beneath.
But Antrax had grown fond of its wronks, which it adapted to serve other needs. It continued to harvest humans for the parts that the wronks needed. It continued to experiment. The humans were no longer intruders; they were prey.
It was the failure of the first energy cell that prompted Antrax to explore the larger world. There were three such cells, vast capacitors that drew their energy from the sun and fed it into the receptors so that Antrax could function. The cells were meant to last forever, so long as there was sun and light. But everything has a finite life, even components that are built to last forever, especially when those components are overworked. Antrax had evolved in its time as guardian of the catacombs. Its commitments to its directive had multiplied, and its hunger had grown. It needed more fuel than anticipated by its creators. Its cells were being drained of energy more quickly than the sun could replenish them. Perhaps it was the strain of maintaining the lasers and probes and wronks. Perhaps the efficiency of the cells had been grossly overestimated to begin with. Whatever the case, Antrax was losing power. It decided that another source of energy must be found. It acted quickly. It sent its probes in search of such a source, far out into the world, beyond what Antrax knew. The probes were not meant to return, only to send the information they acquired. They did as they were programmed to do, and while most places were empty of human life and of the sources of energy Antrax required, one place showed promise. It was across the sea to the east, a land in which humans had survived the Great Wars. Theirs was a rudimentary civilization in many ways, but there were possibilities to be explored. The Old World had changed and Mankind had evolved. The sciences of the past were barely in evidence. Instead, there was a new kind of science. Elements of that science were able to generate power far greater than that which sustained Antrax. The elements could be found in weapons and talismans borne by the descendants of his creators. But genetics and training had infused a few of those men and women with the elements of power, so that in some the power was generated from within.
A dream, or what the dreamer thought was one, had brought the first of the Great War survivors to Antrax thirty years before. Of those, only one was useful. Now that one, supplied with a map that revealed the existence of the catacombs and their contents, had lured others. What had value for the creators would have value for their descendants, whether Antrax comprehended the nature of that value or not. Examined and measured on the islands that Antrax had established as testing grounds through probes dispatched years earlier, subjected to attacks by creatures and spirits no ordinary human could hope to overcome, a few had shown themselves more powerful than their fellows and were therefore suitable for culling. Three at least had come into the ruins overhead, and perhaps more waited without. Antrax would use them as it had used the one thirty years earlier, as components essential to its continued existence, necessary sacrifices to its directive. The creator had been specific. The lives of humans were expendable. It was Antrax who must survive.