The Ardoon King. The Nisirtu

In this sequel to The Apocalypse Script, the Seven Kingdoms of the Nisirtu have destroyed the modern world. There are no more nations or cities. The era of automobiles, telephones, computers, electricity and running water is now a thing of fond memory. The few humans who survived the Nisirtu's wrath are in a constant battle against famine, plague, and anarchy. But the kings and queens of the Seven Kingdoms did not know that a force of even greater power had ensnared them in their own trap.
Now six of those kingdoms have fallen, too. Only the Fifth Kingdom remains, but even that once mighty power has been reduced to a single fortress, Steepleguard, hidden in the mountains above the ruins of Denver. The Fifth Kingdom's plans to rebuild civilization are barely underway when Ben Mitchell, the reluctant Ardoon king, is trapped in a temple of alternating realities. Steepleguard faces a coup staged by a plotting duke from the east.
Annasa Lilitu faces imprisonment as her champion, Lord Disparthian, seeks a mysterious treasure at Denver's abandoned airport. The assassin-queen, Fiela, has her own problems, having led a dwindling army to the wastelands to combat a seemingly indestructible enemy. And yet it is the unseen enemy that is the kingdom's greatest threat.