This is what happens when greed takes over your soul. So there are these gems known as Blood Rubies (better than your old run of the mill rubies, of course) that lie in the hillside under King Gurmuru's village. This once-kind ruler becomes possessed with a need to find every single one so he sends his army out to dig them up, uprooting his citizens and treating them like slave labor. As a result of his impassioned greed he turns into this giant, purple monster that exists in agonizing perpetual hunger. He decides the only way to lift this curse is to gather the dragonballs and wish for the dragon to turn him back to normal. Meanwhile he ignores the fact that searching for the blood rubies is what cursed him in the first place and he grows crueler and more demanding. Enter Goku and Bulma in the first of what ends up being 3 different re-tellings of how the two friends met (fun fact: DB episode 1 retells it, as well as DB movie 4 "Road To The Strongest"). They, of course, want the dragonballs for themselves, which puts them at direct odds with this monster.