The Realm of Monsters by Joharra Harper

The Realm of Monsters is a place where monsters can go and be free to be themselves, without fear of humans hunting them. But their centuries long peaceful civilization is threatened when an enemy of Calista's finds her after a thousand years. With monsters suspiciously at each other's throats, Callista knows her worst fear is happening. Her past has returned for her, Callista, and a group of misfit monsters, must travel to Athens, Greece and reunite with some old friends, as this war they were a part of so long ago, continues. They must stop him from destroying humanity, so that monsters can take over the human world.