A series of 16-cards games, designed by Seiji Kanai and Hayato Kisaragi, running on a variant of the Love Letter Engine: Each player has one card in hand; on your turn, draw one card from the deck, play one of them, applying its effect. At the end of the game, players get to guess in card number order where the Lost Legacy (the '5') is located. Whoever finds it wins.
Cards from different sets are usually interchangeable.
The publishing of Lost Legacy is a bit confusing; since the initial Japanese prints came in boxes with 2 sets each. The games were bilingual. Also, some promo sheets were at times handed out, these were not bilingual. For the English release, AEG decided to release the sets separately.
As game entries on BoardGameGeek are focused on boxes, even if they contain several games, this has caused confusion, as some sets are entered separately, whereas other sets are actually compilations of two separate sets.
The best way to sort out this is to list what sets are available. The Japanese series consists of (with the set names in bold):
Box: Lost Legacy (2012) (contains Flying Garden and The Starship)
Box: Lost Legacy Hundred Year War and the Dragon Maiden (2013) (contains Staff of Dragons and Vorpal Sword)
Promo: The Immortal Heart (spring 2013)
Promo: The Whitegold Tower (fall 2013)
Box: Lost Legacy The Poor Detective and the Castle of Intrigue (spring 2014) (contains Orb of Seduction and Eternal Grail)
Promo: The Mysterious Chest (fall 2014)
There were also some more or less unofficial sets based on the board game indie scene:
Promo: Game Market spring 2014 (spring 2014)
Promo: Game Market fall 2014 (fall 2014)
This list is incomplete, there was a Game Market Osaka version aswell.
There was also an expansion, which is not a set, but special powers for the players:
Expansion: The qualities of an adventurer (spring 2014)
Finally, there was the big box Lost Legacy Legend, that contained eight sets, but where each set was a separate game, and nothing was interchangeable:
Box: Lost Legacy Legend (fall 2014) (contains The Dark Council, Pseudepigrapha, The Truth of All, Li'd Red Riding Hood Sleeps Twice, The Heavenly Feather Robe, The Realm of Anatman, The Werewolf Never Sleeps and The Final Encounter)