Phyrexian Unlife

The cleverest thing this card does is rewrite the loss condition without ever asking you to repair your life total. Normally, dropping to zero ends the game; here, that threshold becomes a second life total that runs from zero downward, and the only thing that can close it out is poison. That conversion is the entire engine: while you sit at or below zero, damage no longer kills you the ordinary way, it accrues as poison counters, and ten of those still ends things. So the card does not make you invincible; it swaps one ten-point clock for another, on a different counter, and hands you the time in between. The design tension is that it answers fast damage races (burn, aggressive curves, anything that wins by subtracting your life) while doing nothing against poison, mill, or alternate-win conditions, which is why it has always lived in builds that pair it with something to seal the deal before the poison clock catches up. Its most notorious partner exploits the implicit promise written into the first clause: if you can no longer lose to having zero or less life, an effect that would otherwise be lethal to its caster becomes a free engine. The card is less a defensive enchantment than a rules toggle, a small piece of text that quietly relocates where your defeat is allowed to come from.