D&Dtools

Benefit

When a foe's movement would otherwise grant you an attack of opportunity, you may give up that attack and instead attack your foe prior to your foe's actual movement. This is akin to a readied action, but Stand Still doesn't affect your initiative count or actual readied actions, if any. The foe must succeed at a Fortitude save against a DC equal to 10 + the damage you deal, or be unable to move into or out of the area you threaten-essentially, this ends the movement of a foe who is closing, and prevents any movement of a foe who is fleeing (if you are normally allowed an attack of opportunity against the fleeing foe). Since you use the Stand Still feat in place of your attack of opportunity, you may only do so a number of times pet round equal to the number of times per round you could make an attack of opportunity (normally just one).

Normal

Attacks of opportunity occur after a foe has already moved within the area you threaten; thus, you are unable to affect their movement with an attack.