Phyrexian Atlas

A three-mana rock that fixes any color is about the most colorless thing a card can be, which makes the Corrupted rider the only reason to reach for this over any other any-color source. Mana rocks tap constantly by their nature: every ramp turn, every activated ability, every time you sink the mana into a spell. Bolting a poison-payoff clause onto that most routine of actions turns an otherwise invisible board piece into a slow-drip source of life loss, but only against opponents already at three or more poison counters. That threshold is the tax the design pays for stapling incremental reach onto a rock that would earn its slot regardless. It rewards a deck committed to proliferate and infect lines, where the counters accrue anyway and the Atlas quietly converts routine mana production into life loss nobody spends a card protecting. Outside that shell it is a plain any-color source, no better and no worse than the rocks that came before it, and that is the logic of the Corrupted mechanic: a conditional upside that costs nothing to include when the condition is met and asks for nothing when it is not. The life loss is small and untargetable, so it never closes a game on its own. It matters most to a poison deck hedging its clock, adding a second axis of pressure so an opponent racing the counters cannot simply stabilize on life and wait the infection out.