Rolto, I would go with folk-lore and gothic horror ideas instead of dnd magic or because "magic", it fits the flavor better IMO. To demonstrate, perhaps your mother was cursed by the vistani when she was pregnant "you shall produce the unclean, that which terrors, that which gnaws and devours. It shall not grow, it will not change unless you repent for what you have done" and she gave birth to a caliban that was a midget caliban that tended to have buck teeth and resorted to canablism. or it was cold and freezing, and it was midnight with a thunder-storm as she gave birth to her child, thunder struck aloud. This being midnight it may be considered an ill oman and the child was cursed due to this. The winter weather made the child frail and weak, His bones to narrow, and enlongated; seeming to be a disjointed, half-shappend figure.
this IMO is much better then "spell x did so"
Also keep in mind the insperations were folktales, quismodo from the hunchback of notre dame and Caliban from the tempest.