Heroes are national units, frequently unique, who appear in the capital by chance. They are frequently quite strong.
The chance of getting a hero each turn is 3%+luck each turn in your capital. At misfortune 3 there is a 0% chance of getting a hero and at luck 3 there is a 6% chance of getting a hero. Note that this is the current luck scale in your capital which may be different from the scales you took during pretender creation if for example an event has modified the scales in your capital.
While most heroes are unique and if lost can not be gotten again (without using a wish) some are not specific heroes but a general class of hero of which there can be multiple, these are known as multiheroes. For example LA Bogarus' s are multiheroes.
Some particularly powerful heroes have a minimum turn arrival limit.