That's bullshit. He fucking grows monsters, not cure cancer. Just because he has the ability to have the ground shit out monsters doesn't mean he knows what the fuck they do, he just grows them. I get it, the king is desperate and is trying every option, and I'm sure the plot will make it so that the sister is cured and is part of the harem, but...seriously? For a 'hero' who cut down a bunch of the trees keeping the world alive, he's forcing the thing he helped in destroying to help him? The king can go fuck himself.