Every pony starts out innocent and good, the still mysterious Alicorn, supposedly named "Opaline" included.

In my AU, Opal is Flurry Heart's younger sister and Cadence and Shining Armor's second child. Opal ends up being the way she is because of intense jealous of her sister and she wanted to be a ruler and in complete control. As a result, she started to slowly divide Equestria apart and sadly, triggered the Equestrian war, that would cause the Unity Crystals to separate after the memory stone was casted, but she was not affected by the memory stone at all like a few select ponies, and the ponies living in cities with creatures all united (That would be Ponyville, Manehatten, and few others)and remembered everything, only the civilians from the three cities of the three tribes were affected, with the exception of the ponies that have familial ties or influences for the Mane 5, they remembered the previous life but would rather shut it away. So, she was unhappy when magic came back to Equestria and wanted to get back what was "her's". She has an unhealthy obsession with the unity crystals.

But when Opal was first born, she was the perfect foal and she was just like any other. Like the saying goes, a monster isn't born, they're created. And that was the case for Opal here.

(Remember, this probably has nothing to do with canon at all and the canonical lore will be very different. Opal may not be the alicorn's name at all. I will edit that when the 8 episode 2nd chapter, Make Your Mark, airs on Netflix.)