This story has gone through a million different iterations and I'm happy with this idea (finally) but would love your thoughts on it!
(The characters are in the comments if you want to know more about them)
The first book takes place in The Morgana School Of Magic, where the most talent young adults from the age of 16 – 19 go to study magic. In this universe, there are no stars in the night sky. The main characters learn that there used to be stars, a whole Zodiac of them created by the goddess Sheena. But one of the constellations, Opheichus, was jealous of Leo's command over the Zodiac. She possessed the body of human with raw magic called Bellua, and used her power to throw the Zodiac from the sky. When the god saw this, they commanded Morgana, the most powerful human after Bellua, to stop her. She failed and was "killed" (she was actually enslaved for 200+ years in a coma). The students learn that the Zodiac did not die, but instead managed to hide in mortals bodies. You guessed it, the students are possessed. They have to learn to get along with their spirits, while also going on a quest to find the remaining 5 Zodiac spirits and the people they've possessed. On the way, they find the 6 towers of old, created by the wanderers who first set foot on the kingdoms. Each tower contains a spell, a spell that when combined together and harnessed by a person with raw magic, gives them enough power to rip through reality and destroy Bellua and Opheichus, and return the stars to the sky.
Sorry, that was really long. Let me know if you think there's anything I could explain better or change, thanks for reading!