テスタメント新妹魔王の契約者 The Testament of Sister New Devil

From the introduction:

Welcome to the world of Shinmai Maou no Testament , a light novel series by Tetsuto Uesu. At first glance this world may resemble the modern one you know, but peer beneath the surface and you’ll find a hidden history of gods, devils, and heroes lurking in the shadows. Human heroes have waged a shadow war against demonic invasion for centuries while the gods callously use humanity as a shield against their foes. But a generation ago this war was brought to a sudden and unexpected end by the efforts of the newly-crowned Demon Lord Wilbert, who negotiated a truce and withdrew all of his forces from Earth.