I have another iterpretation of Balthazar. In my mind, after Isobel dies, Ketheric turns to Shar. He then builds that entire gaunltet of Shar to "raise" dark justiciars. Obviously, thjis is a super human endeavour. So, Shar herself must be involved. This is also the time that I think Aylin was captured (she was already in Eithrym before Isobel died) and imprisoned in Shar's abode in Shadowfell. It was Shar who directly or indirectly imprisoned her rival's offspring for eternal torment. (Why did Selune not interfere ? Selune doesn't seem to interfere with anything actually, e.g. Shadowheart's family, het temple falling apart etc...)
After the justiciars are defeated by the Harpers, Balthazar comes on the scene. To me, he is a Myrkul devout. He manages to revive Isobel and turn Ketheric from Shar to Myrkul. But this leaves Ketheric with a problem. If he wants to keep Aylin he needs to get her out of Shadowfell and in his domain. That is now Balthazar's primary mission, but he cannot possibly do this as he will never be able to pass the 3 trials. When the party arrives that's the reason why he accepts our help. Well, that's how the story goes in my mind at least.
