The THAI PAIN STICK is a wonderfully simple instrument for torturous nipple play.
It consists of two 50 cm long pair of metal rods held together at both ends. Place the nipples between the rods and gradually increase the pressure on them by pushing the rubber rings towards them.
Since the nipples are still accessible, they can be further stimulated, for example with licking, temperature, or more clamps.
If you want an even more intense experience, add weights to the hook in the middle or use it for additional bondage.