Forbidden Knowledge is received at the start Act 2 if you spared Idunna's life during Enemies Among Us.
Letter
Serah Hawke,
Years past, I fell into wickedness and was swept up in the mad ambitions of the blood mage Tarohne. You spared my life at the Blooming Rose, and for many months, I wished you had killed me. But I found the Maker's Light in my deepest despair, and I now seek absolution.
Tarohne wrote many tomes containing dark secrets she learned from the Fade. She protected them with demons forged into their very bindings. You must find and destroy these tomes, lest others follow her dark path. I wish I could help you, but the templars never let me leave my cell. I can only send my prayers.
—Idunna