There are the angels with God in Heaven. The angels that followed Lucifer. This book is about the unmentioned angels whose hearts were torn.
God "The Creator" placed these unmentioned angels in a city He named Adelian, which He placed over the portal to Hell. The Mers (the royal mermaids) were each given a specific power and title over the maids (the servant mermaids). The King and Queen of Adelian went to land to transform into human shape to do a specific mission requested by the Creator. It is only outside of Adelian that they can die; although death is not what it entails with humans. When a Mer dies, they are reborn into the youngest living maid on land. They will not regain their memories of their past lives until they reach Adelian. So the King, the Queen and twin elders are killed and reborn into children and sent to an orphanage. And this is where the story begins.