Vitrouve 3: The Experiment

In the third book all of a sudden, there is a dark shadow creature spooking around in the castle. It seems to be taking all joy from the students, but only Nathan and her friends notice this figure. It seems to follow them and watch them at night. They try to talk to it, but it does not want that. They also keep finding hidden rooms and portals on the school grounds without knowing the darkness they hold. On top of that, their private lives keep getting more complicated because of romances and fights. Someone is also stealing from all the students, but only the small friend group is spared from it, which makes them seem suspicious to the principal. They have to prove their innocence by finding the real thief. They also notice a strange and nervous presence while meditating. When trying to figure all of it out, they get into trouble and almost end up killing someone. All of this is happening while one person in their group is secretly plotting something against them.