Overlord, Vol. 12 (light novel) by Kugane Maruyama & so-bin Book Summary
The Sacred Kingdom has enjoyed a great many years without war thanks to a colossal wall constructed after a historic tragedy. They understand best how fragile peace can be. When the terrible demon Jaldabaoth takes to the field at the head of a united army of monstrous tribes, the Sacred Kingdom's leaders know their defenses are not enough. With the very existence of the country at stake, the pious have no choice but to seek help wherever they can get it, even if it means breaking taboo and parlaying with the undead king of the Nation of Darkness!