Netherese magic. Of what exact form or kind, I'm not entirely certain as of yet - I would need access to research, works and tomes that are vanishingly rare. But it was sealed long ago in that book, likely because they could not destroy it. It was hardly as simply as just turning the cover, such things are enchanted in a myriad of layers of arcane locks and ensealments specifically to keep them closed and yet.

[ he kept opening them. he taps his chest where the mark is. ]

It consumes magic. If I am not able to sate it with a magical item brimming with enough Weave to slack its hunger, it begins next to consume my magic, and then, my vital self. Necrotizing parts of me and my spirit. It is impossible to fully satiate, though I was granted a charm as of late that abates the worst of it.