The Seal of Promise is the result of an event that is activated in the sewer. After meeting both Krobus and Dwarf, head to the sewer to activate a cutscene. In this cutscene, they establish that the dwarves and the shadow people had been at war for a 1000 years, and the war recently ended. The Wizard then intervenes and casts a "Seal of Promise" to keep the two separated. It is unknown how this effects the story or gameplay.