I'm still a bit curious about this sentence. I'm 90% sure I understand what's going on now, but I want to turn it into 100%, because I still think が was a slightly curious choice.
I'm guessing that in this particular sentence, both は and が would make perfect sense, although they have a different nuance (a nuance that admittedly doesn't seem important in this particular instance, but hey
Currently I view the choice between は and が as simply answering questions -- questions that are often implied rather than actually asked. (This idea is explained in detail in the book "Making Sense of Japanese" for anybody unfamiliar with it, although by the time I actually started reading that book, I'd pretty much figured the idea out already. It's nice to have it confirm my suspicions, though.) According to this theory, "ga" answers a question of the form "Nani ga...?", "Dare ga...?", etc.
So then, the sentence クリスタルが アークの ありかを おしえてくれるだろう would appear to be answering the question, 何がアークのありかを教えてくれますか？ (This is pretty much what richvh said in his reply, but rephrased in the form of answering a question.)
By contrast, if the king's statement began with クリスタルは, it would be answering the question, クリスタルは何をしますか？
Both of the questions (and their answers) would make sense in the context, hence, either は or が in the "reply" to the "question" would make sense. Though it still seems a bit odd to me that the king chose to answer the second question instead of the first, considering that he's saying this after he already said "I will give each of you a crystal", which would appear to invite the question "What does the crystal do?" and not "What will tell me the location of the arks?" -- but perhaps it only sounds strange to me because of the order he said the sentences in.
Do I have the right idea here, at least?