738. Legion

738.1. Legion is a Conditional Keyword.

738.1.a. It can be present on Spells, Units, Rune Abilities, Legend Abilities and Permanent Abilities.
738.1.b. It is formatted as "Legion — [Text]".
738.1.c. Starting from the Keyword to the end of the clause, the entire statement is the Legion Ability.
738.1.c.1. On spells and on play effects of permanents, it is functionally short for “If you have played another Main Deck card before this one already this turn, apply [Text].”
738.1.c.2. On other abilities and instructions, it is functionally short for “If you have played a Main Deck card this turn, apply [Text].”
738.1.c.3. This is called the Legion condition.
738.1.c.4. Legion's conditional can apply to passive abilities, activated abilities, spell instructions, or even abilities active in zones outside of the board.

738.2. All instances of Legion on cards a player controls are satisfied by that player playing a single card. Example: One card has three different Legion Abilities. The Legion Text of all three abilities apply as long as one card has been played by the card's controller earlier in the same turn.

738.3. Legion, and whether or not a card has Legion, is a characteristic of the card and may be checked or referenced by other Game Effects.