In the Anglo-Saxon epic Beowulf ,the titular hero's third and final enemy is a dragon, which he defeats at the cost of his own life.
Since I'm trying to tie in "Beowulf" with the sage of Andrew Sutter, whose tale will begin in Escape from the Wastelands and continue through several books, I needed three opponents for our hero.
The first was the warlord Grendel who took his nom de guerre from a partial copy of "Beowulf" he found in a ruin. The second, standing in for Grendel's mother, was a coalition of Grendel's sons waging an insurgency in their father's old homeland.
However, I hadn't fully nailed down the third opponent. I mentioned several possibilities in an earlier blog entry, but I reviewed my "idea file" and those were for later stories that would cover the descendants of Andrew (or his sister Sarah). I didn't have a third enemy for Andrew to die killing.
So I came up with a new one. Decades after Andrew put down the revolt of Grendel's sons, long-sealed passes into the mountains north of the empire Grendel founded and Andrew took from him are blown open and new armies emerge from them to put an end to the post-apocalyptic technological renaissance that two generations of peace had brought about.
These armies are commanded, not by a human enemy, but an ancient artificially-intelligent war machine that had been dug up and managed to take control of the entire region and organize it into its own dominion. Now, using ancient, forbidden weapons (possibly nukes?), it has blasted open the long-closed roads south and is moving out to rule the world.
The notion of an AI using armies of living soldiers is not entirely alien to speculative fiction. The novel When Heaven Fell depicts most of the galaxy under the control of a cybernetic "Master Race" of AIs that entered the Milky Way 100,000 years ago and recruited armies from conquered species, including our own, to expand its control and eventually fight a new enemy that is strongly implied to have defeated them and has now coming calling.
Plus, if it turns out "the dragon" is really something resembling SkyNet, it could provide another hint as to just how a world technologically adept enough to build blacktop roads, use what I've hinted are M1 tanks and Bradley fighting vehicles for war, and revive dinosaurs and create creatures that never existed (the "rippers" occasionally seen in the early chapters of Escape) using genetic technology could get knocked back to roughly (American) Civil War levels of technology.
(Imagine the Terminator universe if the Russian counterstrike on the US destroyed SkyNet's communications links outside its bunker. Cut off from surviving human troops it can deceive or machine units under its direct control, it sits there waiting for centuries until somebody finds the bunker and, not knowing it's there, plugs it back in.)