REFERENCE POST! I’ll put this in the FAQ so people don’t have to trudge through 8 pages of (unfinished) story.
When Candlelight was very little, a powerful witch placed a curse on her. This witch’s name is Bone Cold, the leader of the High Mountain Coven.
The curse was set to sacrifice Candlelight when she turned 19 and a half (The age of adulthood for witches). The curse hit her in the eye and damaged it, which is the reason for the eyepatch in the past.
Over 12 years, the dark magic inside of Candlelight festered and multiplied. By the time she was 19, the magic had reached titanic volumes…But it was still unusable, since Candlelight is not a unicorn.
Candlelight set out on a long journey to remove the curse on her, and she succeeded. In the process, she had to ‘die’ and give her soul away. This process took away her pony life, but allowed her to use the dark magic Bone Cold cursed her with.
Since Candlelight was charged up with so much dark magic, she became a high-ranking demon. She became a succubus because, to be honest, she was always able to charm other ponies very easily.
Now, her eyepatch seals away her demon-form (check the reference page!) and turns her into her (less intimidating) pony form. The patch can be on either side, just as long as it is over one of her eyes. She only eats the lust of ponies, but when starving, POSSIBLY turns to meat-eating.
(Bone Cold is still in power, but is very wary of Candlelight.)