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Space Marine Scout

First strike, vigilance
Concealed Position — When this creature enters, if an opponent controls more lands than you, you may search your library for a Plains card, put it onto the battlefield tapped, then shuffle.

Warhammer 40,000 Commander

First strike, vigilance
Concealed Position — When Space Marine Scout enters the battlefield, if an opponent controls more lands than you, you may search your library for a Plains card, put it onto the battlefield tapped, then shuffle.

Space Marine Scout's triggered ability has an "intervening if clause," which means that it checks to see whether or not an opponent controls more lands than you both when it triggers and when it resolves. If it isn't true both times, you won't search your library for a Plains card.