Use the Sign of the Earth to activate the Pillars of Diamond, Opal, and Amethyst and obtain the Runestones. Place the runestones in the Seal of the Earth to free Blacklash and Hematus. Slay them and return Blacklash's Bindings, the Chains of Hematus, and the Sign of the Earth to Gorn.
The Seal of the Earth is held by three runestones, held in the Pillars of Diamond, Opal, and Amethyst in Lethlor Ravine. Using the Sign of the Earth, you will be able to remove the runestones and unlock the Seal of the Earth.
Be wary though, [player], the black drakes Blacklash and Hematus are not to be trifled with. See them dead and return their bindings to me.
To reach the ravine, travel to the far eastern part of the Badlands. Be careful though, for the ravine is home to countless drakes.