Masa:
Slaves sold to Hellbound became subjects to Beleth's experiments.
You see, when a number of them die, the color of the magic circle changes. When it's yellow, the slaves have a chance of being rescued.
But some slaves may be mutated beyond sanity, so be careful of their attacks!
When the magic circle is in full power, it's red. It produces berserk monsters, but hard as they are to beat, they give some really good rewards!