The Strange Case of Dr. Sparkle and Miss Twilight:  
Miss Twilight is the dark magic side of Dr. Sparkle, created in a combination of magic and potions. The goal was much the same of that of Dr. Jekyll: to rid the world of all evil, starting with the individual evil inside of everyone. However, this magic instead created the personification of evil: Miss Twilight. They struggle for a while, until this final confrontation (depicted here). Dr. Sparkle seems to be holding up at first, but Miss Twilight begins gaining the upper hand, and she tries to show Twilight the door to her worst fears, telling her it is really the door of truth. Twilight does not believe her at first, as she has seen a door much like this one before and she knows that it would make it easier for her Hyde to take over, but she is eventually worn down and takes a look.
 
This is finally finished! I had a bunch of others to pick from (7 others total, 4 completed others), but I eventually went with this one. xD And with only 3 days to spare!
 
This is for the contest held by jekyllandhyde-mlp. The goal was to envision another pony in the role of Dr. Jekyll and Mr. Hyde besides Pinkie Pie, so I picked Twilight. :) The background was slightly referenced from the episode “The Crystal Empire”.
 
The original idea for the crossover of MLP and Jekyll/Hyde was created by Ink Potts on YouTube, or the pairing of assassin-or-shadow and torusthescribe.