In Testament, you progress through consecutive stages dungeon crawl-style, trying to defeat the boss of each stage and its minions. The solo player will have to control all four characters. Each character specializes in an ability (attack, healing, defense), and can be upgraded during the course of the game. Characters form their separate decks of cards at the beginning, and occasionally roll dice for soul and force points.
Bosses have their own action cards, and some of them come with their own set of minions (mobs). You have to beat the boss of each separate stage in order to progress to the next, and finally face the toughest one, Testament. The designer himself warns that the game is difficult to win, and you should not expect to succeed in your first playthrough.