Prism of Souls is a powerful artifact capable of removing the Charmed Ones' powers. The prism is indestructible and can only be kept hidden from the wrong hands.
Marisol Vera hid it in her mirror to keep it safe from those who wish to obtain her daughters' powers. An Imposter Demon posed as Marisol to convince the sisters to procure the artifact for her so they may use it against Harry Greenwood, but its real plan was to steal the sisters' powers. After discovering the truth and vanquishing the demon, the sisters gave it to Harry for safekeeping.