Sanctum Custodian

Damage prevention by the tap, repeatable, with no upkeep cost and no chip to its own life total: that is the quiet ambition here, and the reason the rate looks more useful than it plays. Two damage prevented per turn is a real number against the aggressive decks of an early era, and the ability to point it at any target means it can babysit a creature, a planeswalker, or its controller's own face. The friction is everywhere else. Because the ability is instant-speed, the Custodian can declare a block and then tap to shield itself before combat damage resolves, so the choice is not as stark as the tap line first suggests; the real ceiling is throughput. A single activation per turn scales poorly against any board wide enough to matter: it shields one target for two, not a swarm. White has long offered cheaper, broader prevention in spell form (the Healing Salve family, the various Fog effects), which is the comparison this card never quite wins on rate. What it has over those is permanence: a prevention source that sits on the battlefield and fires every turn instead of a card you cast once and lose. That is the slim niche it occupies, a slow, durable damage sponge for a control shell patient enough to tap it down every turn rather than develop a clock of its own.