Name: Jessica "Jess" Bublanski
Nationality: Swedish
Age: 350 (although, she died at 15)
Class: Infernomancer

Background story:
Jessica was the only daughter of the local landlord, who was very much loved (as much as someone could love a landlord) by the people of the village and only 15 when the Torsåker witch trials in 1675 began. These witch trials are the biggest in recorded history, where on one day 71 people, 65 women, roughly one fifth of all women in the region, and six men, had been beheaded and burned.
She ended up on these stakes because of the local doctor, who was a pedophile. Now Jessica was already quite well developed at her age, and this piqued the interest of our dear doctor. When he confronted her, she somehow managed to overpower him and run away into the woods. Scared that she'd tell her parents and end up being beheaded or hanged, he decided that the best way to deal with this, was to kill the entire family. And as a doctor, he was quite knowledgeable in poisons. So he poisoned the food being brought to the house (the landlord had separate cooks). What he didnt know, was that Jessica still hadnt returned home. She was still in the woods, trying to wrap her mind around what just happened.
When she eventually came home, she found her parents dead at the dining table. She immediately sought help at the local law enforcement (a bunch of riff raff men who thought they were the man in town), who then sought out the help of our dear doctor. Shocked to see Jessica was still alive, he quickly came up with a story that he couldnt find any source for the death of the landlord and lady. And blamed it on witchery, or more precise, he blamed Jessica for being a witch and killing her parents, so she could seize control of the town.
Jessica tried to explain herself, but nobody would believe her, since the doctor was a very respected man. So she was trialled, and quickly sentenced to death by the stakes. And as she burned, she cursed the entire town, every villager, but she saved her strength to curse the doctor. She cursed him with everything she knew, and even some more. She cursed until she couldnt curse no more, and was just spewing blood.
And as the flames rose and started to engulf her, she was confronted by a demon who presented himself as Asmodeus (I really hate using my own name, but he is the demon king of lust, and thats the one I need) and offered her a way out. A way out, and a way to take revenge on everyone and everybody. She could still feel her skin boil and burn, even though time seemed to have stopped. So she agreed. The demon king smiled, and proceeded to rape poor little Jessica, taking her virginity and implanting his demon seed. Jessica passed out during the time, due to the extreme pain.
When she finally awoke, she found the entire town butchered, every man woman and child ripped asunder as if by a bear attack. Only one person was still alive... sort of. The doctor had been stripped of every limb he had, and was rapidly bleeding to death. Strange tentacles in the back of his head prevented him from passing out. Jessica quickly understood that she now controlled a piece of hell, being able to summon forth horrendous creatures. The Jessica everybody knew died that day.
She managed to keep the doctor alive for another 5 months, using witchery beyond anybody wildest nightmares, making sure he felt every second of the pain. When he finally did lose his mind, she cast him out into the abyss of hell, where his mind would be restored, but his body remained broken. Now she roams the earth, just trying to keep out of the hands of the Witchhunters... But thats another story.

TL;DR: young girl gets killed in 1675 on fire stakes, gets 'saved' by demon king, now kills everybody she doesnt like.