Pestilent Souleater

Infect was a black-and-green mechanic, but this 3/3 sells access to it by the turn: pay or two life, and a colorless artifact body starts dealing in poison counters rather than damage. The life payment is the whole pitch for non-black decks, which can borrow the corner of the poison axis they otherwise have no claim to, at a price their own life total covers. That price is also the quiet danger. Against a board that is already attacking you, you are funding your poison clock with the exact resource your opponent is spending to kill you. The grant expires before your next upkeep, so it functions as a per-turn tax to keep the body lethal rather than a permanent upgrade: activate once and the connection is enough, activate twice in a turn and nothing changes, since the counters dealt scale off the creature's power and this ability never touches that. The repetition the card asks for happens across turns, not within them. The five-mana cost and the modest body keep the creature itself unremarkable; the body is just the delivery vehicle, and the activation is the actual product. The Phyrexian symbol in the cost is the bridge that lets any color pay its way into a mechanic that was never theirs.