Sanctum Guardian

Damage prevention is one of white's oldest defensive idioms, but most prevention cards of this era spent mana to wall up a turn: this one trades its own body for the same insurance, and that swap is the whole design. The 1/4 frame is built to survive a combat first, then convert into a single prevention shield the turn it dies, so you are spending a creature that has already done its blocking job rather than holding open mana you would rather use elsewhere. The targeting clause is broader than the rate suggests: it names a source, not a creature, so the prevented damage can come from a burn spell, an activated ability, or a combat blow you see coming, and it can protect any target, including a teammate or a permanent rather than a player. The one-shot nature is what bounds the power. You get the shield once, after the body is gone, so there is no repeatable fog engine here, just a deliberate single answer stapled to a wall that buys time before it cashes out. It reads as a defensive common, but the underlying structure is a held-up prevention spell wearing a creature's clothes.