As its name suggests, the ring of polymorph control is a ring that grants polymorph control when worn. If you are wearing (or have eaten) this ring and something causes you to polymorph, then you will be asked "Become what type of monster? [type the name]". You can then choose to polymorph into a monster. Rings of polymorph control will also allow you to decline a change in form brought upon by lycanthropy.