So, as a fallen Celestial, Zariel can totally pull a Celestial pact to pretend that she's an angel and rope people in. This poor guy had that happen early in his life, and got brainwashed into her Cult and that of Asmodeus. Now he's a cult leader.

Keep in mind he'll have Crown of Stars and probably Investiture of Flame pre-cast. Yes, that means no other concentration spells without losing it.

Since he's a cult leader (possibly still associated with the Bard Out of Hell, above,  he's more of an encounter than just a BBEG. Zombies and such are there as much for him to steal hit points from as anything else.

https://www.patreon.com/posts/58869473