A blood curse. That's what sparked the plague and reduced mankind's population in the city. Transforming the city into a festering wasteland.
The massive land around the Cross mansion created an enclosed sanctuary. A safe haven from the chaos of the plague that tore the city apart piece by piece.
With the rest of the city slowly drowning out the existing life that remained. Humans were the most unfortunate ones in the tragedy. Their bodies were too weak to tolerate the harsh conditions of the city. So most of them were forced to flee.
Looking for a better place to stay. But the such place didn't exist for them outside the city. The government had created an electric wall around the city. Preventing anyone from leaving and anyone from going in.