Safeguard

A brake disguised as a shield. Five mana to cast, then three more every time you fire it, with each activation buying exactly one creature's combat damage for the turn and nothing else. There is no tap requirement, so a mana-flush controller can blank multiple attackers in one combat, but the arithmetic does the disciplining: two attackers means six mana on top of the five already sunk, and against a wide board you run dry of mana before you run dry of targets. That per-use tax is what holds the effect in check. Make it free or cheap and it strangles aggression outright; widen the target from one creature to all and it becomes a hard lock rather than a fog. The targeting parameters (one creature, this turn) keep it priced at attrition rates.

It is also strictly combat-facing. The shield does nothing against burn, ping effects, or noncombat triggers, so it answers precisely the axis a defensive white deck struggles with, the swing-back, and leaves everything else untouched. That narrow scope fit the slower enchantment-heavy white of its era, where a permanent that taxed your mana for a turn's guarantee against the scariest creature read as a fair trade. As built, it is a measured, recurring answer that charges for safety on every activation rather than a wall that buys it once.