Bought an adoptable from  Tetsune! I like her, but I wanted to change a few things So here is my crystal pony unicorn!

Prismia is featured in the MLP book "Twilight Sparkle and the Crystal Heart" where Twilight asks about Cadence's past before she became a princess.

*SPOILERS*
Cadence was originally a pegasus that was adopted by two earth ponies. Cadence was popular and loved by all, except for a evil unicorn sorceress named Prismia, who used the Crystal Heart amulet to suck out the love of all the villagers. Cadence, who was somehow unaffected, confronted Prismia. She then used her love and defeated the heartbroken unicorn with a hug. Cadence was then transported to the sparkly space room that Twilight went too and met Princess Celestia, who told the pegasus that she was a princess because she mastered the magic of Love.
*End of SPOLIERS*

It wasn't described very well in the book, but with my over-active imagination, I replayed the whole scene in my head:

Prismia was the personal assistant of the Crystal Queen, mother of Mi Amore Cadenza. When King Sombra attacked the Crystal Kingdom, The crystal Queen sent Prismia away with the baby princess Cadenza and the crystal heart amulet. The queen is assumed to have been killed in attack and Sombra takes over the Crystal Kingdom. When Prismia and Cadenza were trying flee, they were caught in an avalanche. Just before they were crushed the amulet let off a strange magic, sealing them both away in a crystal orb. Eventually the orb made it to the ocean where it was pushed around in the currents for hundreds of years.

A thousand years later, the Crstal orb spell wore off and Prismia and Cadenza was washed up to shore of Aqua Marerina. A pair of earth ponies found the baby filly and took her home. Prismia, who washed up on the otherside of beach, realizes that she is no longer beautiful and crystally and runs into the forest out of shame. She goes into a long depression about how ugly she has become. Years later, the crystal amulet washes up shore, and Prismia, covered in a black cloak to hide her idendity, takes it.

Cadenza or Cadence as she is called now, grew up realitively normal, with her best friend and neighbor Bubble Lee. The rest is history pretty much...

Prismia belongs to me technically, since this is considered non-cannon
Art (C)  bubblelee