Lightning was the name of a storm lord's weapon. A spear, it sparkled silver and white, almost as if it were made of a jewel.
Once it had belonged to Taranis until he turned away from his epithet 'the Thunderer' and became ever more human in his outlook. It then vanished.
In Swallowing Darkness the skeletal brides presented Lightning to Mistral. It could only be wielded by a storm lord, to touch it without the ability to call lightning would burn the individual, often fatally. The wielder can call lightning to his hand, and can call and direct storms.