BOOK ONE

The sensor array was the size of a very large asteroid or a very small moon, and it had orbited the G6 star for a very, very long time, yet it was not remarkable to look upon. Its hull, filmed with dust except where the electrostatic fields kept the solar panels clear, was a sphere of bronze-gold alloy, marred only by a few smoothly-rounded protrusions, with none of the aerials or receiver dishes which might have been expected by a radio-age civilization. But then, the people who built it hadn't used anything as crude as radio for several millennia prior to its construction.  
 

The Fourth Imperium had left it here fifty-two thousand one hundred and eighty-six Terran years ago, its electronic senses fueled only by a trickle of power, yet the lonely guardian was not dead. It only slept, and now fresh sparkles of current flickered through kilometers of molecular circuitry. 
 

Internal stasis fields spun down, and a computer roused from millennia of sleep. Stronger flows of power pulsed as testing programs reported, and Comp Cent noted that seven-point-three percent of its primary systems had failed. Had it been interested in such things, it might have reflected that such a low failure rate was near miraculous, but this computer lacked even the most rudimentary of awarenesses. It simply activated the appropriate secondaries, and a new set of programs blinked to life. 
 

It wasn't the first time the sensor array had awakened, though more than forty millennia had passed since last it was commanded to do so. But this time, Comp Cent observed, the signal which had roused it was no demand from its builders for a systems test. This signal came from another sensor array over seven hundred light-years to galactic east, and it was a death cry. 
 

Comp Cent's hypercom relayed the signal another thousand light-years, to a communications center which had been ancient before Cro-Magnon first trod the Earth, and awaited a response. But there was no response. Comp Cent was on its unimaginative own, and that awakened still more autonomous programs. The signal to its silent commanders was replaced by series of far shorter-ranged transmissions, and other sensor arrays stirred and roused and muttered sleepily back to it.  
 

Comp Cent noted the gaping holes time had torn in what once had been an intricately interlocking network, but those holes were none of its concern, and it turned to the things which were. More power plants came on line, bringing the array fully alive, and the installation became a brilliant beacon, emitting in every conceivable portion of the electromagnetic and gravitonic spectra with more power than many a populated world of the Imperium. It was a signpost, a billboard proclaiming its presence to anyone who might glance in its direction.  
 

And then it waited once more.  
 

Months passed, and years, and Comp Cent did not care. Just over seven years passed before Comp Cent received a fresh signal, announcing the death of yet another sensor array. This one was less than four hundred light-years distant. Whatever was destroying its lonely sisters was coming closer, and Comp Cent reported to its builders once again. Still no one answered. No one issued new orders or directives. And so it continued to perform the function it had been programmed to perform, revealing itself to the silent stars like a man shouting in a darkened room. And then, one day, just over fifteen years after it had awakened, the stars responded.  
 

Comp Cent's sensitive instruments detected the incoming hyper wake weeks before it arrived. Once more it reported its findings to its commanders, and once more they did not respond. Comp Cent considered the silence, for this was a report its programming told it must be answered. Yet its designers had allowed for the remote possibility that it might not be received by its intended addressees. And so Comp Cent consulted its menus, selected the appropriate command file, and reconfigured its hypercom to omni-directional broadcast. The GHQ signal vanished, replaced by an all-ships warning addressed to any unit of Battle Fleet.  
 

Still there was no answer, but this time no backup program told Comp Cent to do anything else, for its builders had never considered that possibility, and so it continued its warning broadcasts, unconcerned by the lack of response.  
 

The hyper wake came closer, and Comp Cent analyzed its pattern and its speed, adding the new data to the warning no one acknowledged, watching incuriously as the wake suddenly terminated eighteen light-minutes from the star it orbited. It observed new energy sources approaching, now at sublight speeds, and added its analysis of their patterns to its broadcast.  
 

The drive fields closed upon the sensor array, wrapped about cylindrical hulls twenty kilometers in length. They were not Imperial hulls, but Comp Cent recognized them and added their identity to its transmission.  
 

The starships came closer still at twenty-eight percent of light-speed, approaching the sensor array whose emissions had attracted their attention, and Comp Cent sang to them, and beckoned to them, and trolled them in while passive instrumentation probed and pried, stealing all the data from them that it could. They entered attack range and locked their targeting systems upon the sensor array, but no one fired, and impulses tumbled through fresh logic trees as Comp Cent filed that fact away, as well.  
 

The starships approached within five hundred kilometers, and a tractor beam—a rather crude one, but nonetheless effective, Comp Cent noted—reached out to the sensor array. And as it did, Comp Cent activated the instructions stored deep within its heart for this specific contingency.  
 

Matter met anti-matter, and the sensor array vanished in a boil of light brighter than the star it orbited. The detonation was too terrible to call an "explosion," and it reduced the half-dozen closest starships to stripped atoms, ripped a dozen more to incandescent splinters, damaged others, and—just as its long-dead masters had intended—deprived the survivors of any opportunity to evaluate the technology which had built it.  
 

Comp Cent had performed its final function, and it neither knew nor cared why no one had ever answered its warning that after sixty thousand years, the Achuultani had returned.