Yay! 
Here's the first chapter if you would like to check it out: RotG/Frozen: Two Frozen Hearts Chapter 1.
Anyway, here we go with the contest:
Sometime later, in the story, Pitch will take Elsa hostage in his lair and brainwashes her, making her forget her happiest memories, who Jack is, and having her fear her own powers again. While doing so, he turns her into his queen of Nightmares, possessing her.
All you guys have to do is draw and color Elsa from what you would imagine what she would look like as Pitch's Nightmare Queen. You can change her hair color and hair style, eye color, and skin color, and even having her wear a new dress.
You can say either Pitch will change her dress himself while he's turning her, or Elsa will change her dress herself, like she did in the other "Frozen" movies after Pitch turns her.
And you can have her showing that she has black sand or black ice powers, or maybe a combination of both, since Pitch had asked in Rise of the Guardians, "What goes together better than cold and dark?"
Or if you are having trouble thinking about what she would look like, then you are allowed to use something based off of the "Evil Elsa" concept art from the first Frozen movie.
And if you can't draw or color, then you are also allowed to edit a picture, or use the Snow Queen Scene Maker.
And for more help, this is what Elsa looks like as the Snow Queen spirit in Frozen 2, since the story takes place after that movie:
The last day of the contest is on January 31st, the 1st, 2nd, and 3rd place winners will be announced on February 1st, and whoever wins 1st place in this contest, their entry will be used in the story. And as a bonus, I'll draw whoever wins this contest's drawing of Dark Elsa for them and for everyone else to see!
Good luck, be creative, and have fun!
