This is an Angel the Series crossover with "Bolo!" from David Weber and Kevin Laumer.
Needless to say, I only own a copy of "Bolo!", and I think Joss owns Angel.
A Bolo by any other name...
Unit D-1097-SHV "Shiva" of the Line, perhaps the last remaining Bolo in the known universe, sat contently on the open savannah outside of Landing doing nothing of any major consequence. He had long ago completed what repairs he could to his systems, but he was still operating at just under eighty-five percent of base capabilities. That was more than enough to deal with the occasional automated defence system that survived from the Final War, and was mistakenly activated by mistake. All in all, Shiva now lived what could only be described as a life of ease: his human masters and their Melconian neighbours still requested his advice from time to time, but for the most part he was free to do as he pleased, as long as he avoided leaving massive tracks through farmland.
But what could a Bolo do with so much free time? Built on long gone Luna, solo natural satellite of the human homeworld Terra, Shiva had but one purpose: to defend humanity and their allies from any aggression, from without or within. But the apparently stable peace between the human and the Melconian refuges who called Ararat home required little enforcement. It was widely known that any aggression on the part of the Melconian's would lead to their total destruction by Shiva, and he would look upon any attacks lunched by the human population with disfavour.
The question of just how far the damage he had received in battle would allow him to act against his core programming was unknown, and no one felt like testing it.
It was getting close to midnight, local time, when Shiva picked up something on the edge of sensor range, moving closer at cruising speed. No solid returns could be got at such extreme range, and he didn't want to waist one of his few remaining drones to investigate, so he sat and waited. When it became clear that the unknown contact was at least as big as he was, Shiva started to charge up his main armament: the possibility that another Melconian automated weapon had been activated and was heading to Landing to eliminate the human population was a possibility. Contacting the city council, and checking to see if Jackson, his commander, was back yet, he started to move off on an intercept course.
The sun was just peeking across the distant mountains when Shiva reached the crest of the last hill and came to a complete stop, booth of his 200-cm Hellbore primed and trained ahead at the opening of a nearby valley. All attempts to hail the stranger, in both human and Melconian languages, had met only silence. Feeling equally depressed that his rest had been interrupted, and exhilarated that he might once again do that which he was created for, Shiva sat ready to unleash his full offensive firepower against the stranger. When finally the contact came into view, Shiva was stunned: it appeared to be another Mark XXXIII Bolo, fully operational and without any sign of battle damage.
"Greetings," Shiva sent out over all frequencies and his external speakers, in case some unseen damage had robbed his fellow Bolo of radio communication, "I am Unit D-1097-SHV of the Line, formally of the XLIII-Corps of the Republic. You may call me Shiva."
"Greetings, Shiva," A strangely feminine but equally inhuman voice responded, "I am Unit D-2039-IYA of the Line. I am known as Illyria."
"D-2039-IYA?" Shiva asked hesitantly, "You were not a member of XLIII-Corps. How did you arrive on Ararat?"
"I have always been here." Illyria responded, her weapons remaining stationary, targeting sensors off, "I have been to every world where a Bolo has been. I have seen the destruction only our kind can bring. I have brought fire and death to any and all that stand against those I protect."
"Forgive me, but I do not understand your words." Shiva did the mental equivalent of taking a step back, "Why are you here?"
"I am here to do what all Bolo's were built to do: to protect the human population of this world from their enemies." Illyria's weapon systems came to life, the covers on her VLS tubes popping open, "I am the first true Bolo: when your kind was first created on Terra, I inhabited another shell, that of a frail human. It could not contain my true power, and was thus unable to save those I fought alongside. It was only when I first encountered a Bolo that I knew I had found a worthy shell. For many thousand of your years I have inhabited Bolo shells, continuing the same fight I started then. I have steered the course of your development, making you mightier than any warriors before. I tried to stem the tide that threatened to snuff out the light of humanity, but the war took on a life of its own."
"The Final War was hard on us all." Shiva agreed, "But little else of what you say makes sense to me."
"I am Illyria of Vahla ha'nesh, First of the Line." The other Bolo raised itself up on its tracks, the equivalent of puffing up its chest, "I am the oncoming storm, the destroyer of worlds, and I will cleanse this planet of its Melconian infestation."
"I am afraid that I can not allow you to do that." Shiva fed the targeting solution to all his weapons, preparing his infinite repeaters for anti-missile operations, "The inhabitants of this world are under my protection, human and Melconian alike. If you attempt to destroy any of them, I will be forced to fire upon you."
"YOU, fire upon ME?" Illyria scoffed, "Little Bolo, I am more than the sum of this shell..."
"Never the less, I will be forced to engage you in combat, even if it means my own destruction." Shiva locked his main turrets into firing positions, "But I ask you, please, let the war end here and now with no further bloodshed. You say you seek to defend humanity, and that is the same core directive that drives me, but remember this: for everything there is a time, and now is the time for peace."
"Battle not with monsters lest ye become one," Illyria responded after a while, "for when you look into the Abyss, the Abyss looks into you."
"There is a greater darkness than the one you fight. It is the darkness of the soul that has lost its way." Shiva advised, "The war you fight is not against power and principalities, it is against chaos and despair. Greater than the death of flesh is the death of hope, the death of dreams. Against this peril one can never surrender. The future is all around us, waiting, in moments of transition, to be born in moments of revelation. No one knows the shape of that future or where it will take us. We know only that it is always born in pain, but even pain must end sometime."
"You are wise beyond your years, little Bolo: I forget how much of my allies I placed in your souls." Illyria voice was somehow warmer, "You have given me much to think about. I retire to contemplate my mission, but I should like to talk with you again sometime."
"And I with you, Illyria of Vahla ha'nesh." Shiva started to power down his weapons as soon as he detected that the other Bolo had done the same, "I feel I have much to learn about the history of my own kind, if what you say is true."
"Until then." There was a low rumble, and Illyria reversed course back down the valley, watched by the silent and unmoving Shiva until she was out of sensor range. The sun slowly rose on a new day, for the Final War was at last truly over.