Grens were made from the dark hearts. They had bone like skin that could protect them from most melee attacks. They unlike other constructs of the dark could absorb spirit energy. At the same time, they possessed a number of mage powers as well, like the push in the force field, the dark vision of the light field, and voice of the sound field. But they lack the aura of fear that the ghouls and other dark constructs usually possess.
The grens have both male and female among them. The female gren also possess bone wings behind their backs, but these wings only starts to appear after a level of maturation, that is about the time when they produce their own dark hearts.
The female of at least the first, second and third grens can make dark hearts, but the chances of a fourth generation gren producing a dark heart are really low