This is a standalone game, and does not require Aeon's End base game in order to play. However, Aeon's End: Legacy can integrate with the base game for an enhanced experience.
Aeon's End: Legacy is a cooperative deck-building game where players defend Gravehold, the last bastion of humanity. This is a full "campaign", where you'll build your character over multiple "chapters," fighting increasingly powerful nemeses.
At the end of each game, you will choose new upgrades for your character and more powerful player cards to add to the market. At the end of the campaign, you'll have unique characters, new nemeses, new player cards, and new basic nemesis cards that you can play with indefinitely and/or play with all the other Aeon's End content released.