Down-to-earth "property management" shell + inheritance system + epic growth of little people.
Three thousand years ago, humans and monsters coexisted. However, the demon tribe has a long lifespan and a large demand for spiritual energy. With population expansion and urbanization, spiritual energy resources are in short supply, and conflicts between humans and monsters have intensified.
Five hundred years ago, after a "spiritual war" that almost destroyed both sides, the Yin and Yang Realms reached a "Resettlement Agreement" - the demon clan lived in human society in the form of "settlement sites", hiding their identities, abiding by the rules, and sharing spiritual energy resources.
With spiritual veins, demons cannot maintain their human form, and when their spiritual energy is exhausted, they will "lose their shape" - degenerating into the form of beasts and losing their sanity.
Concept: Inheritance system, an epic growth story for little people.