Servant of Earth by Sarah Hawley

In the underground Fae realm, only the strongest and most ruthless have power—but a young human woman forced into a life of servitude is about to change everything.

Kenna Heron is best known in her village for being a little wild—some say half feral—but she’ll need every ounce of that ferocity to survive captivity in the cruel Fae court.
Trapped as a servant in the faeries’ underground kingdom of Mistei, Kenna must help her new mistress undertake six deadly trials, one for each branch of magic: Fire, Earth, Light, Void, Illusion, and Blood. If she succeeds, her mistress will gain immortality and become the heir to Earth House. If she doesn’t, the punishment is death—for both mistress and servant.
With no ally but a sentient dagger of mysterious origins, Kenna must face monsters, magic, and grueling physical tests. But worse dangers wait underground, and soon Kenna gets caught up in a secret rebellion against the inventively sadistic faerie king. When her feelings for the rebellion’s leader turn passionate, Kenna must decide if she’s willing to risk her life for a better world and a chance at happiness.

Surviving the trials and overthrowing a tyrant king will take cunning, courage, and an iron will… but even that may not be enough.