Description from the publisher:
As dire omens fill the skies, demons roam the countryside, and plagues ravage civilization, the Harbinger wanders throughout the realm of Talisman, prophesying the imminent end of the world. If you encounter him, he may help you stave off the end of the world and let you peer into the future — or you may be attacked by demon horsemen and stalked by cursed followers.
Time is running out. When the Harbinger entered the realm he brought with him a prophecy of impending doom and a series of seven Omens to watch for: when the seventh appears, the end is upon you. Talisman (fourth edition): The Harbinger Expansion features four unique decks of Omen cards, each making manifest a different downfall. In one Omen deck the constellations align, threatening the extinction of magic. In another, the dead rise from their graves and battle to overthrow the living. In another, the time of judgement arrives bringing with it war, famine, death, plagues, and other forms of destruction.
In the fourth, the world shatters into many pieces and the terrain transforms unpredictably. New Terrain cards enter the game and alter the landscape, supplanting fields with barrens, deserts with floodlands, and cities with bogs. As doom draws nearer, more and more of the realm is transformed until the seventh Omen appears and all of Talisman becomes a deadly devastation.
In game terms, when the Harbinger is in your region, you must draw from the peril-filled Harbinger deck instead of that region’s normal Adventure deck whenever you are instructed to draw cards. When you draw an Event, the Harbinger moves into your space, but has no ill effects upon you. However, you move into the space occupied by the Harbinger, you must roll a single die. The result could grant you a second turn, or let you look at the top three cards of any deck you choose. It might force you to populate your space with vicious enemies, treacherous followers, and unpredictable events. Worst of all, your result might cause the current Omen to be discarded, activating the next Omen and hastening the advent of the end times.