The Relic of Lore
Part of the The Lore Chronicles series:

In a world of bloodlines and magic, power picks a nobody.
Arabella Garin has spent her life scavenging the radiation-poisoned borderlands, content to stay invisible in the lowest caste of their magical kingdom. But when Queen Isolde dies during the Transfer of Life ceremony, her immense power chooses Arabella instead of the rightful heir—and everything changes.

Thrust from the muddy streets into a glittering castle, Arabella must navigate deadly court politics while ancient magic courses through her veins. Magic she doesn't understand. Magic that could kill her if she loses control. The High Council wants her executed for a royal murder she never committed, and her only allies are a conflicted guard whose loyalty awakens feelings she never expected—and threatens the engagement to her childhood love waiting back home.

With enemies closing in and her control over devastating power slipping, Arabella faces an impossible choice: master the magic that marks her as queen, or watch the last sanctuary in a poisoned world crumble to ash.

But in a queendom built on secrets and survival, the crown comes with a price she never imagined paying.

Can a scavenger's daughter save a realm she was never meant to rule?