Requirement

Within the rules of Magic: The Gathering, a requirement is an effect that forces one or more creatures to attack or block.

Rules
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Keyword
The Mystery Booster test card Ral's Vanguard introduced the requirement keyword, which specifies restrictions on contents of a deck that plays with a Vanguard.
 * A vanguard with a requirement can't be your vanguard unless its requirement is met as the game begins.
 * Your starting deck includes only those cards that will be shuffled to become your library. It doesn't include your sideboard or any other cards that begin the game outside the game.