Mu restraints are ancient artifacts used by The Circle of Thorns, a society whose magic-wielding members are possessed by the spirits of prehistoric savants.
Their preferred host body would be a descendant of their old rivals, the Sorcerers of Mu. The purpose of the trapped restraints is to keep one of those ready for possession. Being possessed is the only way to unlock the restraint. As they channel the life force of the victim they cannot be cut without harming her.
Very few of these artifacts are known to exist. Even fewer are in use. They are varied in configuration: Spreader bar, yoke, tiny thumb- and toecuffs, etc.
The chains are made by binding a minor devil inside them. This devil's desires causes clothing to wither or catch fire and also other sexual stuff.
The victim's active magic is rerouted to the chains. Passive abilities like magic senses are often left usable.
The chains redirect the victim's magic into preserving and protecting her body. She doesn't age, she regenerates damage, etc. Eternal youth at a price.
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The wizard Lillian Mencken has been caught in a spreader bar since the early days of the Midnighter Club. She resides at the Midnighter Mansion and has stubbornly refused any offers from the Circle. Her finely honed ESP abilities have been most useful over the years. Much of the knowledge about these artifacts comes from her.
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The above leans heavily into the lore of the classic superhero MMO City of Heroes. CoH has recently been revived, officially licensed, and is free to play here.