Since in most fantasy campaign, detailed knowlage of magnetics would be metagaming, I find this puzzle a bit out of place and overpowered. Again, I have to ask the question "Why was this made?". Go to Comment
I would like to think that an iconically good creature such as a unicorn would not easily follow this path, even in the case of the violent death of their mate. A subset may fall into darkness, but I would not think all.