Dark Prisoner by Emma Night

It’s been a year since the catastrophic threat of Cornelius was led to rest. All has been going smoothly in Athenai. The peace was going on strong and the security system in place has been buffed up to prevent an invasion from ever happening again. The Royal family welcomes the latest addition to the crown. The heir apparent.
There has been a prison break by one of the oldest Prisoners in the Dungeon of Abyss—called Abyss for short.
What did this Creature of the Nyx want with humans that included mental control? Whatever it was, it wasn’t good and the Royals and Ethereals must band together again to prevent an all-out war with the humans.