Spoiler: this is about the finale — the Restart and what comes after it.
The Rewind Weight
Chapter: The Stillness

After the Restart the gate clock of the Founder's Hall runs on its own, and the Pendulum Bob is free. Read it with the Loupe.
The Rewind Weight gives back the damage of the last five seconds as health and strikes everyone nearby with its swing. A pendulum swings there and back: the weight on your belt remembers the blows, and when they add up to two fifths of your health and you still live, it swings back.
The weight does not undo death: the Last-Minute Watch does that, and you may wear both. After a swing the weight rests for a minute and a half. Damage from players it returns only by half.