Testament of Faith

A blocker that costs nothing when you don't need one. Most defensive creatures occupy a card slot whether or not the board calls for a wall, and a dedicated blocker drawn against control or a slow draw is a dead card. This sidesteps the problem by sitting on the battlefield as a one-mana enchantment, inert and harmless, until the turn an attack arrives; then it converts whatever mana is open into a wall sized exactly to the threat. Holding up three mana stops a 3/3, holding up six stops a 6/6, and the cheap front end means it commits almost nothing to the table early. The instant-speed activation is the whole logic of the design: you set the size in response to the attack you actually face, not the attack you guessed at when you cast it. The cost is that it is only ever as good as the mana you leave untapped, so it competes directly with everything else you might want to do on the opponent's turn, and a wall with defender does not pressure anything back. The animation lasts only until end of turn, so each block is a fresh negotiation with your mana the following combat. It belongs to an early line of enchantments that lie dormant and pay nothing until activated, trading immediate impact for the freedom to spend a slot without committing to a body you might never want.