Most nations have heros, commanders of great reknown and power who have a chance each turn to appear in their nations capital to aid the endevours of that nation. Some of them break nations into new and exciting paths, others are great leader whilst some are just massive brutes well suited to being equiped heavily and thrown into the enemy ranks.
In previous versions of dominions it was wildly speculated that the chance of a hero appearring on any given turn was 3%, +-1% per scale of fortune/misfortune (Fortune influencing it positively and misfortune it negatively), however though it is stated that luck has some bearing on getting heros and merely having stacks of fortune bringers with misfortune will not get one, it is unknown if this is the exact formulae but seems to be a good indicator.
The Worthy heroes mod/s adds more heros to most nations, especially those whom have none, and are generally a staple in multiplayer games. There are two versions, one which is the last version done by the original modder and one which has been continued by someone else.