Maryann was a minor villain from The Fairly OddParents! She is a former godchild of Cosmo Cosma and Wanda Fairywinkle-Cosma.
According to Wanda, Maryann had abused their magical powers. On the 28th July 1914, she wished that Franz Ferdinand (the Archduke of the Austro-Hungarian Empire) and his wife would be assassinated, thus plunging the world into the First and Second World Wars. It was for this reason that Cosmo and Wanda were taken away from her.
It's unknown how she was able to wish that the Archduke would be killed, because in Da Rules, it states that a godchild cannot wish someone dead. Although, it was possible that this rule was created because of her and most of Da Rules are created when a godchild makes a wish that negatively affects the world, such as when Timmy wished it was Christmas everyday.
She was once wished out of Cosmo and Wanda's Hall of Infamy in their house by Timmy after he stole Cosmo's wand. She then tried to steal Wanda's wand and would have succeeded in getting her revenge, but was thwarted by Wanda, and put back into her painting where she belonged.
She is arguably one of the most evil godchildren Cosmo and Wanda ever had and (possibly) one of the most evil villains in the series (next to Future Vicky and The Destructinator), because she actually had Archduke Ferdinand assassined by magic and started World War I, whereas in actual history, Archduke Ferdinand was assassinated by Gavrillo Princip.