A Forgotten Promise (7DS x OC)

(THIS IN AN AU (alternate universe) I don't own any of the characters except for my friend and I's OC)

The Seven Deadly Sins, but once there was nine. A promise made between two good demons. This story is about the eighth sin. Serenity the raven sin of Despond. She can make people feel the pain she does. They'll have no more hope left in them and drives them mad to the point where they pass. She made a promise with an old friend. A promise that wasn't meant to be forgotten, like her.