Long ago several Lords of the Underworld were locked away in a box made of the bones of a very powerful woman. These Lords were the most powerful demons and once locked away their influence on the world lesser. The box was lost for many years. Everyone forgot that it existed. 

Unfortunately the peace didn't last long. One day the box was destroyed. No one knows who or how it was destroyed only that it was gone. The Lords of the Underworld of the underworld were now free. Unfortunately they no longer had bodies. Desperate to stay in the world they were forced into human bodies. These humans woke up one day to discover that they are not alone inside their head. Most of them are decent people struggling to maintain who they were before are despite the demon that now shares their body and mind. 

The Lords are on the surface and they want to stay there. They have no intentions of being locked away again. Their souls are bound with the human so they can't just take over. They have to find away to deal with the human world and stay free.