Festergloom

The color clause is the whole design: -1/-1 to nonblack creatures means your own black board walks through untouched while everything across the table withers. This is a one-sided sweeper sold as a symmetrical one, and the asymmetry is paid for by the deckbuilding tax (you have to actually be mono-black, or close to it, to bank the upside). The damage profile is narrow on purpose: a single -1/-1 mops up token swarms, X/1 utility creatures, and freshly-summoned mana dorks, but it bounces harmlessly off anything with two toughness. That makes it a scalpel for go-wide aggro rather than a board-wipe against midrange. Black has a long line of these toughness-shrinking effects priced as cheap, conditional answers to weenie strategies, where committing hard to a single color turns your sweeper into "destroy all their small things" while leaving your own intact. Festergloom sits squarely in that lineage: not the splashy edict or the unconditional kill spell, but the color-keyed mass-removal that punishes opponents for flooding the board against a black deck that kept its own creatures off the chopping block by simply being black.