Mage Cuffs
Wondrous Item,
Uncommon
(cost 350 gp)
Crafting Components
Cold iron, an opal, and a wire of gold used to pin a lightning bug for display
While you are wearing these handcuffs and cast a spell, use a feature that is magical or psionic, or maintain concentration on one of these, the handcuffs emit a warning whistle as you take 1d6 lightning damage. If the spell or feature requires concentration, you must make a saving throw to maintain the concentration as normal. If this damage reduces you to 0 hit points you are knocked unconscious and stabilized
Source