Overlord Redgit during the Shallows Conquest, which only happens once. The Pure Stone is buyable for 50 Platinum Coins, and increases max health to 1000. Don't worry though, it only takes up two more bars of space on your computer. (Seriously its not that big of a deal). It takes the apperance of a rock with a shining gold heart in the center. Activate it like you would any other potion. He only has 1 per character though, so you cant buy more than one if you have 100 Platinum Coins.