The Rats in the Walls by H.P. Lovecraft

The story is narrated by the scion of the Delapore family, who has moved from Massachusetts to his ancestral estate in England, known as Exham Priory. On several occasions, the protagonist and his cats hear the sounds of rats scurrying behind the walls. Upon investigating further, he finds that his family maintained an underground city for centuries and that the inhabitants of the city fed on human flesh, even going so far as to raise generations of human cattle, who eventually began to de-evolve due to their sub-human living conditions