Polukranos Reborn // Polukranos, Engine of Ruin

The corruption is the whole arc, and this card tells it across two faces. The Theros hydra Polukranos was a black-green devourer that ate the battlefield; here it lands as a plain 4/5 with reach, a wall content to sit back and block. The corruption reads in the activation cost: it carries a Phyrexian white symbol, so the flip can be bought with life rather than white mana. You compleat your own hydra by bleeding for it. What flips is not just a lifelinking body but an engine keyed to Hydra death: whenever Polukranos or another nontoken Hydra you control dies, you spawn a pair of 3/3 green-and-white Phyrexian tokens, one with reach and one with lifelink. That turns the tribe's usual fragility into fuel, since sacrifice and combat trades stop reading as losses and start reading as token generation. The sorcery-speed clause on the transform is the restraint that pays for the payoff: you commit to the flip on your own turn, in the open, and eat a window of exposure before the engine arrives. The card reads as a transforming roleplayer, a defensive body that becomes a Selesnya aristocrats payoff, but both halves are the same design idea seen before and after infection: a hydra that grows by consuming, rebuilt so that what it consumes is other hydras, and what pays for the transformation is your own life total.