Prestige Class or Template?
I like this idea. It has alot of potential, and I think that the progression of abilities is fairly stable. However, I disagree with the idea of the Dying Daughter as a Prestige Class. Perhaps an alternative could be considered, such as a template, much like the vampire template. Here is my reasoning. The starting requirements for the class are a little strange. I don't think it would be an easy task for an underage female to get her hands on knowledge of the planes, (at least to obtain 5 ranks in it), without some serious backstory development (having someone who taught her these things, like a grandparent). Also, most adventurers don't start out at 1st level until they have reached the age of adulthood. The Dying Daughter will need to be quite a prodigy to have gained multiple levels before taking a prestige class, all before maturity. Also, advancement in this prestige class would seem to be something that would be very difficult, since experience gain is based mostly on monster destruction and quest completion, two things the dying daughter doesn't do much of. So, for a dying daughter to reach the 10th level of this prestige class before becoming an old hag is unlikely. Now, you may be asking yourself, how does the Dying Daughter gain her new abilities without a leveling condition. This is where house rules come into play. Let's say that the dying daughter becomes more powerful with each person that she 'avenges' herself with (or every person that she marks that she later kills). Now, by keeping a tally of the number of souls she has tortured unto death, the dying daughter gains further and further abilities from her demonic patron, as the patron would receive great joy from the tortured. In this way the dying daughter is allowed to progress without the need to hoard experience, she is driven only by the one thing that she wants, vengeance. Requirements for the template would remain the same except for skills. The dying daughter would need to be a female of under adult age. She would have had to contact some form of demon and entered into a contract to become a dying daughter. At this point you might include the skill set previously listed as a requirement as a bonus to taking the template. Just something to think about.