A young Opaline is escorted out of the Canterlot castle throne room after after a fierce battle with her teacher.  
 
Ever since watching Make Your Mark chapter 2, I wasn’t quite sure what I wanted to do with Opaline or Misty for that matter. I love them both as characters, but few ideas came to mind until now. For Opaline in particular I had a pretty cliche but plausible theory: She is a former student of Twilight’s who went bad. It’s not the most unique backstory, but it is a very likely one. Based on her dialogue it’s pretty clear that Opaline has history with Twilight, possibly as a close associate.  
 
Opaline was once Twilight’s finest student (Before Luster Dawn) and was exceptionally skilled with magic, leading the princess to took her on as a personal student and possible heir to the throne. She tried to teach her the magic of friendship, but these lessons only served to drive conflict between the two as Opaline believed that alicorns should be feared and dominate their subjects. Since becoming an alicorn, Opaline also developed a lust for magic and power and sought to gain more. Things came to a head when she directly opposed friendship and harmony and attempted to overthrow her Twilight for the title of ruler. Though their battle was fierce and deadly, Opaline was ultimately thwarted. Deeming her too dangerous, Twilight took her powers away and had her banished from the kingdom. Opaline later returned and began influencing the three races against each other, starting a chain of events that eventually lead to the creation of the unity crystals and her second banishment.  
 
That’s the gist of it. Again, it’s kind of cliche but it at least makes sense.