"Here love, this will help you control your dark magic so you don't have any outburst like last time. I know you don't like it but Princess Twilight helped personally with the protection spell so you can be at ease my little night bird.
Everything is going to be okay sweetheart. I promise."

While growing up Cepheus had trouble managing his dark magic and had several accidents. Because of this Luna searched for a way to help him either with a way to control it or a way to repress it.
Thanks to Twilight Sparkle they managed to create a protective horn that absorb the dark magic so Cepheus and the ponies around him can remain safe. Unfortunately it also absorbs his regular magic so he cannot use any of his power while wearing this protective horn. Luna wishes to find a better way to help him...