From the publisher's website:

Myth is a fully co-operative fantasy game. Players take on the role of one of 5 different heroes working together to defeat the Darkness. Each hero has a different skill set which represented by a unique deck of cards. Players’ avatars within a Story are called Heroes. Myth Heroes are clothed in immense power, able to reave all but the mightiest foes. Heroes are controlled through an intuitive Hero Deck. Each deck plays uniquely and allows players more freedom of action and the ability to truly impact the Story each and every Act. Yet, cooperation is the key to victory and bloody fortune. Players must communicate, interact, and assist each other or surely the Darkness will cover all. No player controlling the Darkness. The Darkness is governed by a set of rules unique to each Boss Deck and is activated through Hero actions. If the Heroes get overzealous, then the Darkness pushes back by activating Lairs, spawning Captains, and punishing the Heroes with devious Events. If the Heroes take a more subtle approach, then the Darkness reacts by reducing the potential for treasure and rewards. Without the treasures and equipment to upgrade, the Heroes will find each Act more difficult and eventually fail. The dynamic nature of the Darkness really lends a more fluid feel to the Story. Players aren’t being driven by properties out of their control, nor are they being shepherded by a person whose skill has a direct impact on the enjoyment of the game.