I'm still reading through the Fiendish Codex II. But my first thoughts are that I don't like this book as much as I did with Fiendish Codex I. This one has a lot of contradictory material. Like the thing on the Baatezu heirarchy where they put in non-Baatezu like Imps and Kytons, and ignore the Kocrachons (and Baatezu not in the MM or FCII), exclude the Abishai from that.
Sure they actually came up with good reasons and descriptions for "soul-harvesting" and corrupting mortals. But I feel they messed up in a lot of places, and that's not including neglecting the ancient baatorians, which I was assuming would be mentioned because of the Obyriths in the other book. But then both books are written by different people, so consistency between the two are lost. They could have done something else other than put in 4 more prestige classes and another race.
Also the myth behind the Pact Primeval has far more holes in it than the Demon and Yugoloth myths. This one you could say is only partially true, rather than possibly mostly true like the other myths.
With soul-harvesting in a true neutral society the Baatezu are only going to get 1 soul out of 100, which is why I see them trying to push for lawful evil societies. But when you think about it, the Baatezu have other forces to contend with for souls. The Tanar'i, Deities, Archons and Eladrin are out there to actively thwart Baatezu influences.
Also I'm pretty sure that all outsiders can possess people, I don't know why the Baatezu wouldn't use that as a tactic. They could make a mortal do something out of their control, and get them into trouble, and then face the mortal in person, and offer them advice for getting out of that trouble.
The art gallery has a brand new picture of a kocrachon in it, so evidently the book originally mentioned kocrachons before they were cut for reasons of space.
There is a big problem with the book ignoring every plane other than Mount Celestia, Baator, and the Abyss. The "taint point" system is supposed to decide whether a lawful character ends up in Celestia or the Abyss, but doesn't account for the possibility of ending up in Gehenna, Acheron, Mechanus, Arcadia, or Bytopia. The origin myth really doesn't have a place for CG even existing within it.
I really like the Pact Primeval. I'm fascinated by the idea of mutual contracts between the various degrees of Law.
I assume they don't detail possession in this book because FCI already said just about everything that can be said about it. Most of what's written in the FCI would apply. However, the Pact Primeval prevents baatezu from taking away the free will of others without permission. They could possess only those who sign contracts allowing them to do so. Naturally, they're under no obligation to explain that this is part of the fine print.
As I said, I really like the Pact Primeval. It doesn't have to be ancient and immutable - it can be a living document continually renegotiated by baatezu and archon ambassadors - though I love the idea of the physical contracts existing in the various planes of Law so that if they were destroyed, much of Law would be destroyed too. I can imagine an epic campaign where the baatezu have won the Blood War - until the adventurers manage to shatter Baator's copy of the Pact Primeval, allowing Chaos and the tanar'ri to rally themselves back into balance.