Bitten Apple has put together a set of hidden blades worn by Exio Auditore in the Assassin's Creed series.
Each hidden blade is fully retractable and extends with the flick of your wrist. The trigger is attached to a wire thread that connects to a ring. When the ring is pulled, the blade springs from its sheath.
Each blade is made out of wood, so it won't get you banned from conventions. Likewise, the bracer that houses it is made from faux leather and polymer clay, so don't expect to deflect sword attacks with it anytime soon.
You can order a pair of hidden blades for yourself for $155 from Bitten Apple's Etsy store, where she's taking custom orders for armored bracers and a second set of hidden blades.
