Ebony is a Shadow Being, created by LeSandra for evil purpose. Ebony, however, was not completed, and left on the streets to fade in the light. However, it was not planned for Celestia and Luna to find her. The two sisters hid Ebony in the caves beneath Canterlot and completed her by giving her a heart of light; which, in turn, destabilized Ebony and made her 60x as dangerous.
Ebony is unstable, both emotionally and in physical makeup. She is prone to emotional breakdowns and fizzles in the light (if she stays in sunlight too long, she will disappear forever). Eb, however, only wants to be a hero, but is afraid of facing the world because she might destroy it by mistake.