I'm a Captive in the Realm of Demons

Synopsis

The demons invaded, and humanity lost the war. The male protagonist, Yiye, and his childhood sweetheart, Taozi, were captured and escorted to the demons' cave. In the demons' realm, humans are treated as slaves, used as experimental tools, and even turned into food for the demons. In order to stay alive and escape with Taozi, Yiye used his wits and confronted the four demon lords. In return, a demoness named Megaera offered him her protection on the condition of... having a relationship with a demoness?!