This appears as a magical robe of very fine construction, made of silk and trimmed with expensive ribbons and threads of precious metals. If tested, it will appear to offer the wearer a +3 to AC and all Saving Throws.
Upon being worn into conflict for the first time, however (that is, any time the bonuses might actually be needed), the robe will begin to shrink. It will cause 1d4 HP of crushing damage per minute, with no Saves allowed, and will continue to do so until the wearer is dead.
The robe cannot be removed through normal means, but either a Remove Curse, or a Dispel Magic will allow it to be taken off.
Optional: A magical blade, like a sword or dagger of +1 or better, can slice it off.