Dark Endurance

The cost reduction is the whole design idea, and it inverts the usual math of a combat trick. Most protection spells and pumps reward the aggressor: you cast them to push a creature through, to save an attacker from a bad block. This one gets cheaper only when it defends, dropping to a single black mana the moment its target is blocking. That restriction reshapes the bluff. An attacker who leaves black open has telegraphed nothing an opponent can price around, because the discount does not apply on offense; the mana math only breaks in the defender's favor. Indestructible plus +2/+0 turns a chump block into a clean, one-sided exchange: your blocker survives, the attacker dies, and you have traded a single card to erase one of theirs while keeping your creature on the board. The design tension is that the effect is strongest exactly when you are on the back foot, so it asks a defender to hold up mana and dare the swing rather than spend it pushing damage. It is a rare pump spell built to punish attacking into open mana, and the sliding cost is what makes that punishment cheap enough to keep up every turn.