You could almost call it a pair of handcuffs except the length of chain between each cuff is easily over 10 feet long. The metal chain is surprisingly thin for its length and each link has two barbs protruding from either side.
When two creatures each put on one of the cuffs the chain between them slowly fades until it is completely incorporeal and no longer visible unless you are on the Ethereal plane. While in this state it has no physical bounds and the creatures can move freely. However, when one of the cuffed creatures enters into combat a 5-foot ghostly length of chain appears from both cuffs that is drawn taut towards the other creature. Neither creature can take off the cuffs while the ghostly chains are pulling at them. If either creature ends their turn farther than 15 feet from the other while one of them is in combat they both take 1d4 Psychic damage.
If they are within 15 feet of each other either of the two cuffed creatures can whip the ghostly chain between them as an action. Any creatures hostile towards the cuffed creatures that are directly inbetween them must make a DC 16 Constitution saving throw. On a failure, the creature takes 1d10 Force damage and is Restrained until the end of their next turn. On a success, the creature takes half as much damage and is not Restrained.
If either of the cuffed creatures falls unconscious or takes off their cuff the chain suddenly snaps out of the Ethereal plane at the feet of the other creature and the other creature takes 2d6 Slashing damage. If they are farther apart than 15 feet the other creature takes an additional 2d6 Slashing damage.