Razor Pendulum

A finisher with a trigger that only wakes up once the game is nearly decided. The threshold of five life is the whole design: the artifact does nothing while a player is healthy, then turns each of that player's own end steps into a two-point clock once they have dipped low. The structural quirk is that it reads everyone's life total, not just an opponent's, so a player who has spent their own life on fetch effects, painful mana, or aggressive trades can find this thing pointed back at them. It also stacks: more than one copy means more than one trigger per end step, and the damage itself can push a player further below the line on a subsequent turn. What it cannot do is start the work; something else has to bring a player to five first, which makes this a closer rather than a threat, a piece that finishes a race the rest of the deck has already won. The colorless cost is the only thing that lets it slot into a deck that wants reach without committing to a damage color, but the rate is slow and conditional enough that it asks the rest of the deck to do the heavy lifting before it earns its keep.