Sleep Magic

The one-mana pacifying Aura has always come with a catch, and here the catch is spelled out on the third line. Tapping the creature on entry and denying its untap step is the classic soft-lock that reaches back to the long line of Aura tappers: a way to strip a blocker or attacker out of the game without killing it. What distinguishes this build is the escape hatch. Any damage dealt to the enchanted creature sacrifices the Aura and frees it, and that clause is the fair-trade term against the aggressively cheap rate. Most permanent lockdowns of this kind either cost more or hold firm against everything short of removal; this one hands the opponent an out that lives inside the ordinary flow of a game. A ping, a fight spell, a burn effect, even the opponent's own damage-based triggers can all break the enchantment. So the card poses a sequencing question rather than a deckbuilding one: lock the threat when the opponent has no clean way to route a point of damage back into it, and you have removed a creature for a single blue mana. Read it correctly and the lock is durable, because a tapped creature cannot block and cannot be attacked directly, so incidental combat damage is off the table; the danger is a burn spell or a self-inflicted ping the opponent still controls. A tapper priced like a cantrip, honest about the fragility that keeps the price fair.