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.
The Nightsong is
locked in a soulcage, and the reason Ketheric is immortal is necromancy as supplied by Balthazar as an intermediary for Myrkul. When we fight him, he doesn't actually have Myrkul amplified extraordinary powers. This is the danger of using combat stats to determine what Act 2 figures are capable of in lore. It just doesn't work. Everyone has either a MacGuffin, is powered/de-powered by a major deity -- but, only when convenient -- to the point you can't assign "power levels" reasonably. It seems Selune lost track of Aylin until she prayed to her after being freed, which leads to her regaining some (all?) abilities. Aylin's abilities seem less innate than bestowed upon by Selune. Unless, what actually happened was some sort of healing magic. In parallel: Isobel was also lost to Selune, so some people speculate the shadow curse "blinds" Selune to the happenings in the region.
The reason Balthazar wants to move the Nightsong is, to my understanding, because Ketheric will be marching on Baldur's Gate. This is apparently a problem, though I fail to understand why. It's arguably the best place for her to be where she is now, but the spell may have a "range".
