Starting Chains by Nicole C. Luttrell
After years of war between Montelair and Septa, the two thrones are united by family. Victor's nephew, Morgan, is sharing the throne with the last heir of the royal line, Jacob. He and Lenore decide to travel to Montelair with their newborn daughters to help broker peace.
But peace among their own people is harder to achieve. The city is tormented by a terrorist who calls himself The Tinker. He and his group of anarchists plant bombs through the city and call for the death of the new kings from every street corner.
Meanwhile in Calistar, Sultiana and Devon are marching to war with Kussier. The ancient hatred between the two countries is sprung anew when Sultiana is declared heir to the Calistar throne.
Waiting at the border, though, is a much darker enemy. A force from legend threatens to consume both countries, and possibly the world.