Servant Of Earth - Sarah Hawley

Kenna Heron is a servant in the faeries' underground kingdom of Mistei. Known in her village for being a little wild, Kenna must help her 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 them both.

With only a sentient dagger of mysterious origins, Kenna must face monsters, magic, and harrowing tests . .
But worse dangers wait underground, and soon Kenna gets caught up in secrets and passion. With time against her, Kenna must decide if she's willing to risk her life for a better world and a chance at happiness.
Surviving the trials will take cunning, courage, and an iron will . . .

but even that may not be enough.