Heart of the Deicide
The Storm Dragon’s frozen heart, and his greatest weakness. The only way to stop him permanently is to destroy his heart. It is a powerful artifact in its own right.
Long ago the Storm Dragon was but a mortal servant to the dark powers. While still mortal, he was ordered to turn himself into a Lich. Ever faithful, he did so, and created this as his phylactery. As his power has grown, so has the power of this item, but even so, it still serves as his greatest weakness.