Breastplate of Shapechanging

This breastplate is decorated with fur and other bestial features.  While wearing it, you gain the following benefits:  

• You can cast alter self at will.    
• The armor has 3 charges.  While wearing it, you can expend 1 charge to cast the polymorph spell on yourself.  It doesn't require concentration, and instead lasts for the full duration.  You can choose to end the spell early as a bonus action.  The helm regains 1d3 charges at the end of a long rest.  If you have spell slots of 4th-level or higher, you can cast polymorph using one of those spell slots.      
• You gain proficiency in Constitution saving throws.  If you already have this proficiency, you instead gain a +4 bonus to Constitution saving throws. 
• While transformed by the polymorph spell, you have advantage on saving throws against being paralyzed, poisoned, or stunned, and you have resistance to acid, cold, and poison damage.        
• While transformed by the polymorph spell, bludgeoning, piercing, and slashing damage from nonmagical attacks is reduced by 1d6 + your Constitution modifier.  
• If you are transformed, the armor merges with your alternate form, but does not visually appear on your body.  It magically embeds itself under the skin of your new form.  While transformed, you may add your full Dexterity modifier, instead of to a maximum of 2, when determining your AC.    

This armor consists of a fitted metal chest piece worn with supple leather. Although it leaves the legs and arms relatively unprotected, this armor provides good protection for the wearer's vital organs while leaving the wearer relatively unencumbered.

Notes: Proficiency: Constitution Saving Throws