Dwindle

The -6/-0 number is the giveaway: this is a Pacifism variant that takes the long way around, dismantling both halves of combat by two different mechanisms rather than switching them off with one clause. The power penalty is large enough to neuter almost any attacker (six is enough to zero out the vast majority of creatures), while the second clause closes the obvious escape hatch: a creature you have declawed offensively can still throw itself in front of an attacker, so blocking is answered with destruction rather than left as a free option. That pairing is the design idea. Most Aura-based combat locks answer one phase and leave the other open; this one turns the attack step into wasted mana and turns the block step into a death sentence, and the parenthetical (the attacker stays blocked) means the enchanted creature cannot even trade its life for a favorable exchange. The cost of that completeness is that it is conditional in a way hard removal is not: the creature still taps for mana, still carries its triggered abilities, and a bounce or blink reclaims it whole while stripping the Aura. It is a containment effect, not an answer, and it sits in blue's long preference for pinning a threat down while leaving it alive, the color paying for its refusal to destroy things outright with the risk that a single flicker or return-to-hand puts the problem back.