Blowfly Infestation

The condition is the whole pivot: a creature dies while bearing a -1/-1 counter, and another such counter lands wherever you aim it. That structure is a chain reaction waiting for a spark. Seed the first counter on a token or a one-toughness body, let it die, and the new counter moves to the next victim, which dies, which fires the trigger again. Given a way to place that initial counter and a board of small creatures (yours or an opponent's), the loop walks across the table on its own. The card never names that loop in its text; it simply rewards a play pattern built around -1/-1 counters, which is what makes it a payoff rather than an engine starter. It does nothing in a deck that has never touched a wither or persist effect, and a great deal in one built to feed it. Persist creatures are the cleanest fuel: they return to the battlefield already carrying a counter, so the moment they die again the infestation fires and hands a counter to the next target, keeping the deaths coming. The "if it had a -1/-1 counter" clause is the entire balancing act, gating what would otherwise be an oppressive every-creature-death trigger behind a specific commitment. You have to earn the chain by building a board that already carries the counters; absent that, the enchantment sits inert, a payoff with nothing to be paid off by.