I really went with a more general approach to this.

In a nearby valley, close to Pure Heart, there is a legend of a special sword. One that if wielded by the chosen one would grant them the ability to command anything and anyone. Rumours claim that Kraesartos could command even the rocks to move, albeit very limited. However, there is a catch; in exchange for all that, the wielder must always burn half of their food in a fire every meal time. Otherwise.... A nasty curse may await.

I've seen this happen to a wielder. They had purchased the sword from me, and they had been eager to try it out once it showed affinity. But right then it was meal time, and so absorbed were they that they forgot the warning I had given them.

They sold it back to me the next day with a terrified look in their eyes, only saying Ruby all over again. This only fueled my desire to find out more about the Ruby Pure Heart, already known to the world as an enigma. Until then, this will be in the anti-magic sheath I have, never to be used...