In kingdoms with a high Culture aspect, heroes will have access to the "Chosen by the Watcher" self-interaction. This makes them "The Chosen One" for 6 hours, granting them +25 Focus. It takes an unknown amount of time for the self-interaction to "recharge."
To get the achievement "The Chosen," all ten heroes must experience being "The Chosen One" at some point. It's unknown if all ten heroes must experience it in one kingdom of if the experiences can be spread out (i.e. 5 of the classes being Chosen in one kingdom while the other 5 are chosen in another).