Your expert shot renders your foe unaccountably clumsy.

At-Will   ✦     Arcane, Weapon
Standard Action      Ranged weapon

Target: One creature

Attack: Charisma vs. AC

Hit: 1[W] + Charisma modifier damage. The first time the target misses with an attack before the end of your next turn, it falls prone.
Level 21: 2[W] + Charisma modifier damage.

Published in Arcane Power, page(s) 6.