Silken Noose

Speak true or it tightens — the cord knows when you lie.

A fine cord tied into a noose, tough enough that only a bronze blade will cut it. Untied and left unattended, it reties itself.

When it is looped around a neck, it tightens whenever the victim lies or refuses to answer, strangling them until they die or tell the truth. Trying to remove or loosen it makes it strangle; it only slackens once the victim is dead.