The putrefax is a large living mass of putrefaction that has arisen from Rey-Goor, the swamp at the edges of the Tangle. It is comprised of decaying matter, including rotten gelfruit among other things, black sludge, tissue, and metal. Copper-scrap "teeth" protrude from its mouth. The putrefax has been corrupted by Phyrexian influence.
It is a shambling, unstoppable abomination.