The Guns of Avalon picks up directly after the first Amber story. Our hero, Corwin, walks through shadow yet again, gathering an army so that he may mount a second assault on Amber in service of defeated Eric and taking the throne for himself.
But all is not well in Amber, and by extension, in shadow. Amberites are able to utter a curse with their dying breath, or with an equivalently dire moment. When he was punished and imprisoned for his failed coup in Nine Princes in Amber, Corwin uttered such a curse, willing Eric to never find comfort nor peace so long as the crown sits upon his head. That appears to be working way better than Corwin intended, as a strange black road now cuts through multiple shadows, much the same way that the princes and princesses themselves travel through shadow. Dark things walk the black road, as if a shadow-spanning highway weren’t cause enough to fear about the state of things. Sure enough, the black road is the result of Corwin’s curse, manifesting his hatred for Eric and a determination to destroy his reign. Unfortunately for Corwin, it may prove to destroy Amber itself, regardless of if Corwin retakes the crown or not.
Corwin has a secret weapon, however: guns. Gunpowder famously does not burn in Amber, but Corwin has found a substance that does. With it, he intends to raise an army with a massive technological advantage over the Amberites, and pay Eric back in kind for his cruelty in the previous book.
Much like the last book: read it.
