Darkness by Harry Turtledove Books in Order

The death of a duke prompts Algarve's king to reclaim a lost duchy, drawing neighboring countries into war. Alliances, nationalism, and hatred widen the conflict in a fantasy world where magic and military power threaten ordinary lives.

Algarve's magical war grows more brutal as Kaunians are deported and killed to power its attacks. Rival nations seek weapons of their own, leaving civilians and soldiers trapped in an escalating struggle where military strength increasingly depends on mass death.

Algarve continues conquering territory using magic powered by murdered Kaunians, while refugees seek uncertain shelter. Scholars in Kuusamo investigate a bloodless alternative, offering hope of changing a war whose violence reaches soldiers and civilians across the continent.

As Algarve advances through Derlavai with lethal battle magic, scholars in Kuusamo and Lagoas work to develop a different source of power. Soldiers, refugees, and civilians confront choices that test their humanity as the war approaches a decisive stage.

A magical ice ship gives Algarve's enemies a way to carry dragons into battle and challenge its advance. As the tide begins turning, hunted Kaunians and other civilians still face slaughter in a war fueled by stolen life energy.

The Derlavaian War reaches its final battles as rival allies already maneuver for the peace that might follow. Escalating magic and shattered personal lives complicate the struggle to defeat Algarve, with victory offering no guarantee that conflict will end.