Monster girls are strange and wondrous creatures that fascinate the humans they share the world with, though humans often forget that monster girls find them just as fascinating. With many of the same features as human women, monster girls are divided into numerous breeds based upon all of the ways that they differ. But a common thread unites them. From the littlest Dormaus to the mightiest of Dragons, they seek one thing above all else: love.
While human souls are believed to suffuse their flesh, the souls of monster girls live outside their bodies in indestructible spheres about the size of a marble. Primitive men used to say that to hold a monster girls' heart was to hold dominion over her. This is inaccurate. To be sure, holding a monster girls' heart puts her at your mercy, but the soul within her heartstone will truly submit only to the one worthy of her love, her bond-mate.
And if all you seek is dominion, that's probably not you, dickhead.
Monster girls' hearts are always open as they search for their bond-mates. Sadly this means they have no defense against the cruelty of man, and when such a man holds their heart for too long, the trauma can weigh upon her soul, creating darkness where before there was only light. If she can endure no more, she becomes a creature of chaos and hate, her only desire to hurt those that have hurt her.
Such are the Tenebrae, pity them.