Mithril Coat

The flash and the auto-attach are what separate this from every other protection Equipment. Most gear that grants indestructible sits on the battlefield telegraphing itself, waiting for an equip cost you pay at sorcery speed on your own turn. This one holds up: cast it in response to a board wipe, in response to targeted removal, in response to combat math turning against you, and it arrives already fastened to a legend, dragging its own indestructibility along so the Equipment itself survives the same sweeper. The legendary-creature restriction on the enter-the-battlefield attach is the price of that ambush: it is not general-purpose protection, it is a bodyguard for whichever commander or key legend the deck is built around. That restriction narrows the target but does nothing to blunt the timing, and the timing is what recommends it over a Darksteel Plate or a cheaper hexproof effect. The equip cost still exists for reattaching later, once the ambush is spent, but the card is designed to spend most of its life resolving as a surprise rather than being paid up front. It is the rare piece of protective gear that answers a threat on the stack instead of anticipating one, and that instant-speed window is the difference between insurance you buy in advance and insurance that pays out the moment the disaster lands.