Impromptu Sneak Attack

Beginning at 3rd level, an arcane trickster can enter a special stance that allows him to improvise a way to sneak attack targets regardless of their awareness. An arcane trickster can maintain this stance for one round per day. For every two levels beyond 3rd, an arcane trickster can maintain this stance for one additional round, up to a maximum of 4 rounds at 9th level. The target of impromptu sneak attacks loses any Dexterity bonus to AC, but only against those attacks. This power can be used against any target, but creatures that are not subject to critical hits take no extra damage (though they still lose any Dexterity bonus to AC against the attack).