Prison is an adventure map object, that holds an imprisoned hero. When a hero visits the prison they will free the imprisoned hero who then joins the player, free of charge. If the player already has eight heroes outside of towns, the prisoner is not freed. After the imprisoned hero has been freed, the prison will disappear. The imprisoned hero can be set by a mapmaker or be chosen randomly. A random hero will have a default starting army.
In Horn of the Abyss, two additional variants of prison were added: