Morningstar Book One

Nord Morningstar becomes the new vessel of the Hollow, a magical parasite with an insatiable hunger for magic. To protect her sister, she willingly accepts to be banished to the world of Nyu and is fated to carry the Hollow until she dies. But this ultimate sacrifice carries a forgotten, cruel cost - a pact with a demon. Don't Forget, Morningstar! Available on RoyalRoad:
https://www.royalroad.com/fiction/73643/morningstar-book-one-complete