Waterknot

The blue answer to a creature that does not kill it. Where black prices out hard removal and red points damage at toughness, blue's color-pie deal is that it rarely destroys; it locks. This is the clean, durable form of that bargain: the Aura taps the creature on the way in, then the second clause makes the lock hold by denying the untap step indefinitely. The result functions like a permanent tap-down, neutralizing an attacker, a blocker, or a tap-ability without ever sending it to the graveyard. That distinction is the whole tradeoff. The creature is still on the battlefield, still a target for the opponent's pump or recursion, and because the Aura is anchored to an enemy body, bouncing or blinking that creature undoes the lock and leaves the enchantment in the graveyard: a clean one-for-one exchange, but one the opponent gets to choose the timing of. It is also, like every Aura pointed across the table, a tempo gamble: an answer that goes nowhere if the threat leaves before it resolves. The lineage runs back to Pacifism's design problem, a long line of tap-down and pacify enchantments each searching for the right balance between cleanly stopping a creature and leaving an escape hatch. Waterknot's version splits the difference by shutting the creature down entirely rather than just barring attacks or blocks, which makes it the better answer to a problematic activated ability and the worse answer to a threat that can shrug off being kept tapped.