Bloodchief Ascension

A one-mana enchantment that does nothing the turn it lands and then asks you to wait: three quest counters before the engine ever fires, each one gated behind an opponent losing two or more life in a single turn. That ramp-up is the entire balancing act, and it is also the card's whole problem, because the front half of the cost (accumulating the counters) and the back half (the payoff) were designed for the fair grind of a creature-combat format, where chip damage and incidental drain slowly tick the quest forward. The payoff is what broke. Once it has three counters, every card landing in an opponent's graveyard can drain two life, and the game has a long history of effects that bury an opponent's library wholesale: a heavy mill spell or a self-replicating mill engine converts that trigger into a string of two-life drains that closes out a game fast, the life gain attached to it merely a courtesy. The tension is that the two halves were never meant to be solved separately. The trigger checks any loss of life, not just damage, which means the moment someone realized the counters could be loaded outside of combat and the graveyard filled by your own hand, the patient attrition reading evaporated and what remained was a repeatable drain bolted onto other people's mill. A slow-burn payoff redesigned, by its own players, into a combo finisher.