With her weird relationship with Rasticore and the Septarian statues in St. Olga's, I think she's either Eclipsa's descendant (whether because Eclipsa was already pregnant with a child from her husband when she left, or has monster ancestry) or a former queen or princess who opposed Mewni's treatment of monsters/however it was connected to Septarsis, and was erased from its history after being banished or leaving.
How this relates to her brainwashing fixation I'm not sure. Obviously she would no longer want to associate herself with Mewni but there's still some backstory we're missing here.
Also there seems to be a connection too between her level of technological advancement in St. O's and that of the Septarian characters. One group could have given it to the others.