Royal Assassin

The original tap-to-kill creature, and the card that established a design pattern Wizards has refined over the decades: a fragile body whose threat is procedural, not combat. The early-era rate is generous in a way modern design no longer permits, three mana for a repeatable creature kill at instant speed (the tap is not restricted to your turn, so the assassination can land on the opponent's attack step, during their upkeep, or while a pump spell sits on the stack). The tapped-creature requirement does all the balancing, and it makes the card play like a soft Propaganda: an attacker tapped to swing in is fair game, so the threat of activation taxes every aggressive turn. Decline to attack and the creatures stay untapped and untouchable, which is the deal the card offers (a deterrent that levies a toll on combat rather than a free executioner). That asymmetric pressure (the threat of activation doing more work than the activation itself) is the design idea later cards inherited and constrained. Visara the Dreadful pushed the rate up but kept the tap cost. Murderous Rider folded the effect into a body with an escape hatch. Royal Assassin's descendants all pay something it did not: a higher cost, a tribal restriction, a one-shot clause, a requirement that the target be attacking. The 1/1 body was the tax charged instead, and across much of the game's history it was tax enough.