Nimh's grandmother was a sorcerous. Before Nimh was born, her grandmother made a deal with a demon. In order to gain power from the demon, she had to pay a price. The price that the demon required was that her granddaughter be cursed with visions of the future upon birth. The grandmother made the bargain, sealing her grandchild's fate before her birth.
Doesn't sound so bad, right? Except that the visions are hardly coherent and often involve catastrophic events that she cannot prevent. The visions extract a lot of energy, weakening her, and bares a lot of similarity to seizures when she has them.
And so, without further delay, here is Keli's take on our dear Nimh: