I'd like to get some inspiration for my campaign if possible. We've recently finished forge of fury and are running into Red Hand of Doom (loosely-just using them as a general guide). The first few levels before forge of fury were homebrew and the characters started out in a nearby city.

After the downtime following forge of fury, one of my players expressed a desire to have an internal good/evil struggle by coming across 'evil' magics (a wizard). He visited the arcane academy in the city, failed to find much, except something, buried, forgotten and untouched for centuries - an apparently evil, but seductively powerful boom - found him. He was transported to another plane where a huge looming figure faced him, and only nodded, upon which he was sent back with a new fascination with the book.

The book started to gradually reveal itself to him, some dark spells included, and some weird effects on the character. In our first session kicking off red hand of doom, the wizard showed this book to the party paladin who...well you can guess what happened next.

The party did everything they could to destroy/get rid of the book with the wizard protecting it. They eventually bagged it, sent it off with the parties giant owl mount and dropped it in a nearby river - with the wizard left unconscious and apparently unable to wake up.

The player then made a new character - another paladin with an oath of vengeance, sworn to find and kill this wizard - who the party currently have hidden in the upstairs of an Inn.

My original plan was for the book to eventually reveal a recipe to become a lich and continue to lead the wizard to lichdom and evil deeds, though this all happened much sooner than expected!

Where would you take it from here? An opportunity for the party to travel into the shadow fell, confront the devil and bargain for his soul back? Or a convenient NPC villain, sent back to find the book and reek havoc and vengeance on his once party?

Linking into red hand of doom would also be super useful. Any ideas much appreciated!