Ages ago the world suffered a massive earthquake, splitting the earth's crust. The hole revealed a giant cavern deep beneath the surface. First came the creatures that flew, then the ones that crawled. Then, they found people.
Below, they called the world Microsha: a bastardized word of the native peoples meaning 'the end of water'. The cultures of the surface exchanged many things over the years: language, food, art, technology, politics, and (as history always dictates) diseases.
The scientists and investors of the surface harnessed a disease that allowed the infected to drastically reduce aging. This was the Serum, and it changed the world. For hundreds of years, the Serum was spread like a vaccine to death. Everyone could become, essentially, immortal.
Yet, not everyone could afford to live forever. Riots across the globe caused the Serum factories to be destroyed, leaving the world – above and below – without it. The few who could obtain a dose of the remaining stock took theirs early or were killed for their vials. People were frantic for a substitution. They found one: blood.
Microshan "witch doctors" could take the blood of those who had taken the Serum and work magic upon it. Those who could afford such blood drank it eagerly, wishing for immortality like that of vampires in folklore. The rich keep living. The poor keep dying. Those who sell their blood, do. Those who can keep "bleeders" in slavery, do.
That is about to change. There are lights on in an old Serum factory. Someone is trying to bring back the Fountain of Youth.
Perhaps, they will succeed.
Perhaps, the rich leeches will stop trafficking blood.
Perhaps the poor will prosper.
Perhaps, it's all too good to be true.