A wizard/professor named Lavenza Hale pulled a Shape of Water with an eldritch horror "for science" and got pregnant. She removed the fetus and grew it in a test tube for nine months. Thus, Mary Hale was born.
Mary foiled all of her mother's attempts to hide her from the world (it's hard to keep a glow-in-the-dark anglerfish tiefling secret). Along the way, she made a lot of friends through her simple zest for life and open-mindedness. Despite the subliminal images of abyssal horror she impressed into people's minds, she was a very good kid.
At age 10, Mary witnessed her mother summoning her eldritch parent again. Professors don't get paid much, and raising an overactive tiefling isn't easy. Rather than feeling fear and existential horror at meeting her other parent, Mary proceeded to berate her parent for not being present in her life or supporting her. After 10 minutes of this, the eldritch horror was so annoyed they contemplated wiping themself from existence. Fortunately, they realized their progeny would make an excellent harbinger. Anyone who could argue with a being of pure chaos and win would wreak havoc on the planes for sure.
The abyssal terror ended up paying child support through their pact with Mary: she would finish school and act as a harbinger of chaos, and they would teach her how to use her dark inheritance and be more present in her life. For the next eight years, Mary worked her buns off to complete a full academic degree by age 18. Armed with a fancy piece of paper, no practical life skills, and the powers of the Abyss, Mary's ready to take on the world and do some good. She has to sow discord and anarchy in the world in the world, but that's not necessarily a bad thing. Right?
LEVEL 3: Went to a weird future-dimension (Ithaca) and Chuult
LEVEL 4: FINALLY learned how to summon a fiend dog familiar, Cotton Mather. Won a "Goth-Off" in a party of four warlocks and a sorcerer.
LEVEL 5: Fought some mummies and read half the Book of Vile Darkness