Pitch Black, Nightmare King
I really enjoyed the movie ROTG but the books-- Guardians of Childhood, by William Joyce---are truly wonderful! Plus, William's illustrations are so gorgeous, and although his books are 'for kids' I am enjoy the heck out of them.
Toothiana, aka Tooth Fairy
If ROTG movie had not been for kids I would of liked to see some truly horrific nightmares come out of Pitch.
Honestly, we all have had a nightmare about our teeth, so this would be Tooth's worst fear.
So I decided to do another Toothiana.
This is a melding of the movie and book versions. I really enjoy both designs.
Seriously, if you have not read the books go do it! Guardians of Childhood books are really freakin' good, I loved them more than the movie.
Jack Frost
Do not worry Jack. I will not let you be alone ever again.
Look, amongst the waving kelp your new friends will keep you company.

I would suspect that Pitch would have a special Nightmare for Jack.
(the image is rough but if i work on it anymore i fear i might throw it away)
North's Nightmare
I believe that Pitch would be specially cruel to North. Taking away his precious pets, beloved sleigh, and wonderful eyes.