Ouija boards can be used (sometimes unknowingly) to open a portal between the hereafter and our mortal plane. If said portal is opened, the person who opened the portal runs the risk of opening themselves to the Spirit Realm and thus, a dark, demonic entity can be free to take advantage and enter their body/aura.

That nun, being a close minded religious sort, didn't know or care to know and destroyed the Ouija board thus preventing closing the portal that may have been opened. This allowed the demonic entity free reign over its victim.

This is why you must cast a circle of protection before opening portals to the spirit realm to contain any malevolent entities from entering your home, as well as protective spiritual wards on yourself to prevent anything from entering your body/aura. Always close the circle of protection after properly closing the portal so that any residual spiritual energy can't escape.

Believe if you will, or don't. It's up to you. I'm not here to convince you. Just to offer my two cents.