From publisher blurb:
A Tale from the Past
Centuries ago a group of adventurers belonging to a holy order of clerics and paladins chased a demonic entity, known as the Whispering Demon, to the complex of subterranean caves located at the center of the Frozen Marshes. The clash between the adventurers was brutal and deadly, and the combatants fell one after another against the demon. Only one of them was still standing to face the demon in a last deadly clash. The holy warrior was about to deal the fatal blow to the demon, who in return, realizing that his demise was inevitable, cursed himself and the body and soul of all his chasers. The whispering demon's body burst and was consumed by his own unholy fire until only his heart, made of a pulsating black rock and violet amethyst, remained in place. The body and the soul of the adventurers were consumed in the explosion and returned as undead under the service of the Demon's will, with the demonic heart becoming a bridge connecting his layer of the Abyss with the Prime Material Plane. Now an Abyssal Mist surrounds the caves, with the frozen marshes around it making the landscape even more impassable.