I am trying to remember the name of a science fiction novel in the eighties that developed over three stages. Magicians summoned djinn or genies that were really technology from the past like tanks, nuclear bomb etc...
There was also a character who couldn't be attacked because damage directed at him was inflicted by him.
You're asking about Fred Saberhagan's 'Empire of the East' series.
The first novel, The Broken Lands, features the Elephant, a tank from the old world. The third novel, Changling Earth, has the demon Orcus that, in the old world, was a nuclear bomb caught mid-explosion.
Apparently there is now a fourth book, which I haven't read (Ardneh's Sword, published 2006)
