Moonstone, a mysterious stone with no detailed description in the novel. Sometimes the moonstone was termed as a part of a dead moon's core. While somewhere it was mentioned that every moonstone once held the essence of a moon God. While some believed moonstones were used to store their God's faith, prayers and powers, to be given as a blessing to his most devout followers in their time of need.
The truth never clarified as the author clearly didn't put so much focus on an artifact when he had to write a shit ton of others too.
But one thing that the author highlighted every time was that the moonstone could attract the light of the moon and was best to be used by those who had moon gods as their backing.
In the novel, this same frighenart forest was used for the exam ground for the protagonist and his classmates too.