Eragon

When he asked Glaedr, the old dragon said, There is an invisible poison here, in the air you breathe, in the ground you walk upon, and in the food you may eat and the water you may drink. The spell will protect us against it.

What … poison? asked Saphira, her thoughts as slow as the beats of her wings.

Eragon saw from Glaedr an image of the crater by the city, and the dragon said, During the battle with the Forsworn, one of our own, an elf by the name of Thuviel, killed himself with magic. Whether by design or by accident has never been clear, but the result is what you see and what you cannot see, for the resulting explosion rendered the area unfit to live in. Those who remained here soon developed lesions upon their skin and lost their hair, and many died thereafter.

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It is not well known, but even the smallest speck of matter is equal to a great amount of energy. Matter, it seems, is merely frozen energy. Melt it, and you release a flood few can withstand.… It was said that the explosion here was heard as far away as Teirm and that the cloud of smoke that followed rose as high as the Beor Mountains.

Pg 498, amid the ruins, inheritance