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.