Text (4th/8th/9th/10/M10): 1/1. ; {Tap}: Destroy target tapped creature.
If the creature is no longer tapped (it untaps by Twiddle or other effect) when the Assassin is resolved, then the ability is countered. [D'Angelo 2000/03/03]
You can target creatures you control. [bethmo 1994/05/01]
Cannot target itself because you choose the target at the same time you are tapping him. At that time he is not yet tapped. [D'Angelo 1995/07/18]
Extended tournaments (see Rule 913) banned this card from 1999/10/01 to 2005/08/20 because it was not in a legal set.
Note - This card was of creature type "Assassin" and not "Human Assassin" until Ninth Edition and Oracle 2005/08/01.