Potion of Mithrilization

Wondrous Item (potion), very rare

This potion gleams with silver light, reflecting and multiplying even the faintest source.  

As an action you can pour this potion on any creature or object. If it is a creature, it must succeed on a DC 17 Constitution Saving Throw. If it fails, or it is an object, it is transformed as per the True Polymorph spell into an equivalent weight of mithril, arranged in a latticework of bars and threads in the approximate size and shape of the original target. Mithril has an AC of 21, and the object has hit points equal to the maximum range for objects of that size listed in the DMG.