By Foreshadowing That He Is Not Completely Satisfied With Cedric’s Evil Plans And That He Thinks Evil Sorceresses Are Really Cool - Blog Posts
Queen Miranda, Mistress of Evil
The story behind this is that when Halloween rolls around, on the third year of her being queen, Miranda decides that she really wants to dress up this year for Halloween, because she hasn't yet since coming to the palace. Back in the village she always used to dress up as a villain, but she was not sure if that was still acceptable now that she had become queen. Of course, as soon as she mentions this idea to her family, they told her that they thought it was a fantastic idea, and she should go for it! In fact, maybe she should even “take over the Kingdom” for Halloween (A.K.A. themed Halloween party where she's in charge)! So she made her own costume and the rest is history.
Wormwood can't talk yet at this point in time, but everyone could tell that he wanted to be her evil sidekick for Halloween, so he's acting as Diablo to her Maleficent. (Even though he knew this whole thing was fake, Wormwood still very much enjoyed this because Maleficent is his favorite villain of all time and Miranda is surprisingly good at acting in character).