Legacy games are board games whose rules and components change over time based on the outcome of each game played and the choices made by the player(s). They will oftentimes make physical changes to the board game by, for example: marking the cards, placing stickers, destroying components, opening sealed packages, and so on.
The changes made in a Legacy game are designed to be permanent. A Legacy game's full experience is played out in a campaign that can only be played once. To replay the full experience again, the player(s) would need to purchase another copy of the game or, if available, a "recharge pack".
Official and unofficial methods to circumvent Legacy mechanics are common in popular Legacy games.