Golgari Raiders

A printed 0/0 that resolves into nothing without a graveyard to feed on: cast it into an empty yard and it enters, then dies to the same state-based check that kills any creature with zero toughness before you ever untap. That is the bargain the undergrowth counters buy back. The count is fixed at resolution rather than granted as a static buff, which means the size is locked in the moment it enters: exile or shuffle away the graveyard afterward and the body keeps every point it arrived with. Haste is what turns that snapshot into a threat, since the whole payoff of converting dead creatures into a large attacker collapses if the thing has to sit through a turn cycle waiting for summoning sickness. Attacking the turn it resolves lets it dodge the sorcery-speed sweepers and removal that would otherwise pick off a fresh threat at end of turn. So the counters set the size, and haste sets the timing; together they make a creature that only exists at full value once the early game has been spent filling the yard with creature cards, then cashes the whole pile in for one sudden swing. It is a payoff card in the strictest sense: the reward scales exactly with how much you have already lost, and it does nothing at all for a board that has been keeping its creatures alive.