Instant, 1R (2)
Common
Target creature can't block this turn.
Draw a card.
"Fortunately, merfolk aren't used to opponents with feet."
—Gerrard of the Weatherlight
Illus. Terese Nielsen

10/4/2004: Must be used before blockers are declared in order to affect blocking decisions. You can't wait to see what your opponent declares and then try to stop them.
10/4/2004: You can use it after combat or even on a creature which can't possibly block this turn because it's that player's turn to attack, but it generally has no effect other than to let you draw a card.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander
Legal in Modern