Quag Sickness

The scaling is the entire point, and so is the friction built into it. A removal spell that reads -1/-1 is filler; one that reads -5/-5 or worse, but only in a deck willing to commit its manabase to a single basic land type, is a payoff handed out for color discipline. It belongs to black's lineage of "kill it by shrinking it" effects, which sidestep regeneration and indestructibility the way outright destruction cannot: a creature reduced to zero toughness is put into the graveyard as a state-based action, not destroyed, so neither keyword saves it. What narrows the bargain is that it counts Swamps specifically, not black mana or black permanents. A deck splashing black off duals and fetches gets a sorcery-speed enchantment that does almost nothing, while a deck running a dozen Swamps gets a clean answer to nearly any creature for a single application. As an Aura, the effect is bound to its host: it dies with the enchanted creature, goes to the graveyard rather than re-attaching, and is vulnerable to enchantment removal in the meantime. That fragility is consistent with the rarity and the bargain. This is a swamp-count reward built for the most basic mono-black shell, asking only that you mean it about the color before it pays you back.