Bonds of Quicksilver

The flash clause is the whole reason this design works as a tempo play rather than a clunky four-mana lock. Tapping down a creature permanently has a long pedigree (frost effects and the various Sleep-style enchantments), but most of those are sorcery-speed commitments that telegraph themselves a turn early. Casting this in response to an attack turns it into disguised removal: once the creature is tapped from swinging, the Aura keeps it locked down, and it never untaps again. The catch is that it answers the body without answering the permanent, so anything that cares about a creature being on the battlefield (a static buff, an enters-the-battlefield engine, a death trigger you would rather not hand them) keeps ticking. It does nothing against a creature already content to sit back, and an opponent can bounce or sacrifice the enchanted creature to shake the Aura, taking the two-for-one with them. There is also a timing wrinkle worth naming: because Bonds of Quicksilver does not tap the creature on entry, dropping it on an untapped blocker leaves that blocker free to keep blocking; the trick only bites once the creature has tapped itself attacking or activating. Its best use is against a single threatening attacker or a key tapper or mana creature you want neutralized without feeding the death-payoffs that hard removal would. The instant-speed enchant is the load-bearing piece, letting a control deck hold the answer alongside counterspells and deploy it only once the opponent has committed.