Not long after the death of her older sister, Bethany, Assbutt found herself resorting to blood magic as means to keep her family safe (and we all know how sucessful that was, don't we?) Her deal with the desire demon dictates that in exhange for all the benefits of essentially limitless power, she would have to trade the one thing she valued the most, which in this case was intimacy. As a result, should Assbutt become intimate, it will be seen as 'borrowing' from the demon and afterwards will have to 'pay back' the demon with her own lifeforce. Although not nessicarrily fatal, this would either render Assbutt near-dead or kill her. Another stipulation of the contract is that during this time, the demon lays claim to Assbutt's body should she die or be 'close enough' for the demon to succeed. Assbutt can theorhetically prevent this in such an event, but it would be a great feat of mental strength.
Another condition of the demon's contact is that every full moon, the demon is allowed to take full posession of Assbutt's body for one day, but is not allowed to cause any damage to Assbutt, and is not allowed to kill or harm those she cares for. This day is typically a chance for the demon to lure a few non-magi into the fade, mess with the chantry and update her facebook.
Not that this is purely a plot device or anything. Because it's not. You suck.