Arianna Ortega was the primary antagonist of the Dresden Files novels Changes. Though she recieved only a minor mention in a previous book she was quickly propelled into being one of the most monsterous antagonists in the series.
Her first truly evil act that is known is purposefully marrying the European Conquistador Paolo Ortega purely to break his heart.
Her second (and truly horrendous act) was intending to sacrifice Maggie Dresden, an innocent 8 year old. The reason was to avenge her humiliation by Harry and his grandfather even though she detested Ortega (who Ebeneezer had killed). She had Maggie kidnapped, arranged the slaughter of her foster family, and than arranged a massive Red Court Ceremony at the temple of Chichen Itza. The intention was to sacrifice Maggie persoally and unleash a curse that would kill her family (two people). Despite this, hundreds would have to die.O.
Realizing that Harry could challenger her to combat she launched a "peace mission" purely to keep Harry from getting backup and to cripple the red court's foe the White Council.
Unfortunately for Arianna, Harry was able to deduce the location of the sacrifice (with the help of allies, minion carelessness, and other factors) and gained the power needed to face the Red Court by selling his soul to Queen Mab. He arrived at Chichen Itza and secured an audience minutes before the sacrifice. The Red King decreed that the sacrifice would be delayed in order to ensure a clean fight (since Harry's claim was as legit the Red King had the excuse to let Harry go first). and both Harry and Arianna were to report to the duel arena. Arianna cooly admitted that she simply viewed sacrificing Maggie as "Buisness" and was more angered by Harry's purposefully choosing armor of the conquistadors.The two faced off and while Arianna was able to blast Harry against a wall his heavy armor and durability and quick thinking allowed him to weather her assault.
Harry ultimately won the battle thanks to the Red King purposefully choosing weapons that would give Harry a fighting chance, causing Harry's foe to try to repeat the same move twice, creating an opening for Harry to attack her. He impaled her with numerous spears, including one the size of a man's fist. As Harry approached to deliver the death blow Arianna could only stammer in shock and horror as her plans crumbled into ruin around her. She mocked Harry calling him cattle, to which Harry only mocked her with a simple moo. He than delivered the final blow, destroying her. Her plan not only failed, but sealed the doom of the Red Court when Harry turned their own spell against them (Since they were all a massive blood line thanks to sharing the Red Kings blood).