Let's talk about Legacy game mechanics. Traditionally, when you are done playing a game, all the rules and cards and components stay the same. The next time you play, you are playing the same game. Everything resets. The character that was the murderer in the last game still shows up to dinner when you replay the game. However, what if the next time you play the game, something is changed. That though you are playing the same game, a component has changed. Like the country Spain on the board is now called something different? Or that a card you used in the last game has been torn up and no longer usable in the game. That is legacy.
A little history
Legacy Games all happened when game designer Rob Daviau jokingly asked why the murderous characters in Clue were asked back to dinner in a meeting. Then he designed Clue Legacy, but that idea was rejected when he pitched it to Hasbro. But then later he was asked to design Risk Legacy. He followed that up with Pandemic Legacy: Season 1.
What is it all about.
A legacy game is meant to be played in many sessions in a campaign and each game play is different from the previous session. Changes that are made are permanent like placing stickers on the board itself or rules are changed. Sometimes you can write on the cards or put stickers on cards to change it. Other times, a card is destroyed or taken permanently out of the game, and then you open a new pack of cards and add them to the game when they are unlocked.
Each copy of a legacy becomes unique to someone else's copy of the same game. In Gloomhaven, there is a board of the map of the world and you place stickers on it to denote your progress along this map. Also stickers are placed on some cards giving them new ability or rule. If you feel uncomfortable making permanent changes, you can always purchase removable stickers for particular legacy game.
A problem with Legacy games is that it take a large amount of time commitment to play wth the same players over time. These aren't like games you just play for an hour and you're done. You can play for 2-3 hours or more. Then you play again with all new changes and to continue your campaign later on.
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List of Popular Legacy Games
Pandemic Legacy: Season 1
Pandemic Legacy: Season 2
Ultimate Werewolf Legacy