This is the chain you shackle to your other leg when you're about to go to sleep. You are the only one who is able to unlock it.
Both chains are totally unbreakable, and their anchors are immovable from the wall, due to a mysterious juju enchantment. All juju enchantments are equally mysterious. Each is more equally mysterious than the last.
Sleeping with shackled legs doesn't make for very comfy slumber. Still, it's advisable to stay in the habit, unless you want all your belongings to get messed with.