In the adventure the Aamuel head is trying to absorb the Hethradiah head by absorbing "a demonic soul supercharged with life energy". Namely a half-fiend great wyrm red dragon named Ashardalon who has bound a living balor into his body to replace his heart, which was torn out by a foe. It's a pretty damn cood scenario.
I could easily see a disaffected Cathezar as a prisoner in Into the Maw or an agent of Malcanthet in Wells of Darkness. Nurn is even easier to work in since the Savage Tide is Hethradiah's pet project.