Stumbling Strike - EX

You can use your attacks to stymie an opponent’s movement instead of directly attacking. When you successfully use a weapon with the reach weapon special property to strike a target with an attack of opportunity, instead of dealing damage to that target, you give the target the flat-footed condition and reduce all the target’s speeds (including burrow, climb, fly, and swim speeds) by half; both effects last until the beginning of the target’s next turn.