This appears as an expensive necklace of gold and jewels, with an embossed image on a central pendant of a smiling face. It will radiate magic if tested, and appear to be magicked to allow the wearer to speak and understand any spoken language.
Upon first being used when it really matters, the wearer will still be able to understand any language, which could be useful, but will only speak in curses and insults, no matter what they try to say. The effect doesn't occur when using a language the wearer actually knows, but if they attempt to say anything in a language provided by the necklace, it will come out as nothing but foul words and caustic phrases.
If the wearer, or someone else, attempts to take the necklace off, it will constrict, and try and kill the wearer. Once they stop trying to remove it, the necklace will loosen again. Attempting to break or damage it gets the same result. A Remove Curse spell will be required to get this piece of jewelry off the victim, but that will not get rid of the curse on the necklace itself.
After revealing its true nature, the smile on the face of the pendant becomes a frown.