Dark Sphere

The math is the joke. Sacrifice this for its single use and you halve exactly one source's incoming damage, rounded down, so that any odd number leaves a point bleeding through: against a three-damage source you stop one, against a one-damage source you stop nothing at all. This is prevention designed to feel like a hedge rather than a wall, and the zero cost is the tell. Wizards was experimenting with artifacts that asked for no mana commitment but paid back almost nothing, the deliberately marginal counterweight to the unrestricted power of the early Moxen. The tap-and-sacrifice clause caps it at one shot per copy, and the "source of your choice" clause means you pick the threat with full information: you fire this in response to a spell or after attackers are declared, knowing exactly what is about to hit you and choosing to shave half of it. It survives now as evidence of a design language that had not yet decided "free" needed to mean either negligible or broken; here it just means small, awkward, and honest about it.