A Thunder-Wyrm is a long, finned serpent, as long as three wagons end to end, slender for its length, with a narrow head and a mane of filaments along its spine that stand out with charge. Its hide is dark violet-grey, like storm cloud, and lit from within along its whole length by branching blue-white veins that blaze when it feeds. It swims the air in long rippling curves, and when it enters a bolt it vanishes into the light and comes out at the top of it. It makes a deep thrumming that can be felt through the roar of the thunder.

They live in the clouds over the Storm Heart and come down along the bolts. A storm holds a few.

They feed on lightning, gulping energy from the bolts as whales gulp water, and are not hunters of flesh. But they follow the bolts, and the bolts follow movement and warmth, so a Thunder-Wyrm often comes down on the very traveller the lightning was going to strike. A traveller in its path is struck by the bolt and the wyrm together.

They swim through lightning, take its energy into themselves, and can release it in a strike of their own when threatened. They float in the storm’s reversed patches as easily as fish in water.

Nothing certain is known. Smiths' tales say the young are born in the strike of a great bolt, and fall back to earth as small glowing eels.

A Thunder-Wyrm’s filaments, taken from the rare dead one, are prized for storm-steel. They are dangerous mainly by accident, to anyone the storm is aiming at.

Plains folk see them as the storm’s serpents and say thunder is their voice. Some cults of the sky hold them sacred and pray to them for rain; they bring only lightning.

The plains songs tell of the hunter who brought down a Thunder-Wyrm with a copper-tipped spear driven into the ground in its path, and was burned black from the shoulder to the hand for it.