Fred Saberhagen's Swords multi-series (the First, Second, and Third Books of Swords, and the eight Books of Lost Swords) take place in a fascinating world of technology and magic that he originally created in Empire of the East. Some background:

Fifty-two thousand years prior to the saga of the Swords, there was an enourmous, world-ending nuclear catalysm. The Earth and the human race were saved by Ardneh, a mysterious semimechanical pseudogod. Ardneh's Change immediately replaced technolgy with magic. A few bits of tech continued to work, such as the everburning lamps that continue to glow for fifty millenia. The largest impact of the change was the conversion of atomic explosions into demons. Although demons plagued humanity ever afterward, they had to follow Saberhagen's rigorous rules of magic just as humanity had to, and demons, although dangerous, are not unkillable city-destroyers.

Two thousand years before he Age of the Swords, Ardneh battled Orcus, the king of the demons, in a war for dominion over the Earth. In that battle, both Ardneh and Orcud were annihilated. This began the end of the Change, restoring technology to a magical world.

But wait.

When the gods were born, seemingly out of nothing, at the time of the Swords' creation, the Change clearly was not gone.

Characters from Empire of the East who return in the Swords books include: Draffut, Beast-lord and avatar of life, who is not a god, although at one time he was a dog; and Wood, lieutenant of the evil John Ominor who nearly took over the world.

The other important forces in the Swords' world are the Temples (the Blue, lords of wealth, the Red, hedonists and whoremasters, and the White, servants of Ardneh, healers) and the Emperor, who may or may not be the creator of the universe.

In the First book of Swords, "In what felt to him like the first cold morning of the world, he groped for fire." So does Vulcan appear, building his smithy in the throat of a volcano, wielding Earth-fire and sky-iron to create the Twelve. He tempers the blades in the blood of six unwilling helpers, and gifts Townsaver to the seventh, Jord, after taking his arm, with instructions to hold it in trust for his eldest son. That night, Jord's betrothed sleeps with a masked figure who she believes to be the local Duke. It's not as big a deal as it may sound. It's a funerary rite, where all unmarried are free to reaffirm life in the most elemental sense. BTW, that Duke? Ain't.

Years later, Jord's youngest son has the sword thust upon him, and the adventure truly begins.