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As a standard action during its turn each round, the creature can literally run over an opponent at least one size category smaller than itself. The creature merely has to move over the opponent. The trample deals bludgeoning damage, and the creature’s descriptive text lists the amount.
Trampled opponents can attempt attacks of opportunity, but these incur a -4 penalty. If they do not make attacks of opportunity, trampled opponents can attempt Reflex saves for half damage. The save DC is 10 + 1/2 trampling creature’s HD + trampling creature’s Strength modifier (the exact DC is given in the creature’s descriptive text).