The past history of the player.
A data store for the player’s accomplishments, including achievements, heroes, and equipment. Anything that can be unlocked is notionally stored in the player’s Legacy, and the Legacy can be queried during the game for purposes of unlocking content, providing relevant legends, etc..
Heroes are added to your legacy when they retire, when you build a tomb for them, or when they survive a victorious campaign. Legacy heroes can be recruited in subsequent games instead of new random farmers by paying extra legacy points depending on their legacy tier.
The higher a hero's legacy tier, the more abilities they can come back into the game with.
When you win a campaign, you can choose one of your heroes from that campaign to promote to the next legacy tier. Their new items, history, and abilities will go with them.
When you have brought a legacy hero back, new changes to them will be saved only if you promote them at the end of the campaign. Otherwise, they stay the way they were before in the legacy.
If you have old Folk Heroes in your Legacy you are no longer interested in, you can browse to them and remove them by clicking "Forget". Once you have promoted them to Local Legend, forgetting is removed as an option but you can still mark them as not playable.
Non "playable" characters are not offered as options to recruit, but may in the future be able to be encountered during events (this feature is discussed by the devs but not yet implemented).
The elemental weapons you unlocked in a victorious campaign are also added to your legacy, and are available for crafting in future games.