Author Chapter 5.3 (Yuusha, Yamemasu)
So her plan is to bring peace through mass brainwashing, achieved with an artifact that may or may not actually be able to any of what she wants, and her attempts to acquire it have lead to her forces being gutted.
It's a bold strategy. Let's see how it works out for them.

Ten bucks says that any land she controlled in the demon world has been taken over by others.
How would a philosopher's stone lead to world peace though? I'm imagining it as an artifact with massive amounts of "power". In which case, is power all that's needed to bring peace? If so, then just recruit the hero.