Per the description for them:

"

Long before men walked upon Elemental, dragons filled the skies; for thousands of years, the Nine Broods dominated the world. They dug great caverns in the mountain ranges of the north and west; built burrows of unimaginable size in the deserts and jungles of the south and east. Then came man; then came the armies of Al-Ashteroth, bent on supremacy, seeking to obliterate any who stood in their path. Individually, the dragons were more than a match for any man; yet even their greatest broods were helpless against the overwhelming numbers of warrior-priests sent against them from the south. Only a few dragons survived the slaughter; they retreated to the fringes of the world, where they remain still.

The Ashwake is the mightiest of those, a dweller of deserts and wastelands, a breed of dragon immune to fire, immune to magic... a creature of near-unimaginable power, and with a ferocious hatred of mankind.

"

 I see that they are in fact not immune to magic. They are vulnerable to cold actually. They do have high spell resistance, but that's not the same thing as immune to magic. After a few shadow bolts, high magic resistance is at least manageable. Immunity would make them even tougher. I suppose it might be difficult to make them immune to magic and vulnerable to cold at the same time though. 

Regardless the description should probably be modified to "resistant to magic" at the very least.