Myrkenstone

A large chunk of the Dragon Star, which had been sent by the evil god Gyphon. When the Dragon Star disintegrated over Gron, Modru was able to use the Myrkenstone to generate the Dimmendark, an ever-expanding zone where the sun is unable to touch the Foul Folk. This allowed him to invade without fear of the Ban destroying his troops.

However, the Myrkenstone was destroyed by Tuck Underbank, who fired the Red Quarrel at it.

Trivia

 * The substance that the Myrkenstone is made out of is never identified.
 * It is unknown whether there are other pieces of the same material elsewhere in Mithgar.