Crown of Roses by Hillary Raymer

Cursed with the blood of her enemy, Maeve will stop at nothing to prove she’s worthy of her mother’s crown. Her opportunity finally arrives when the Scathing, a dark fae magic, afflicts her kingdom and leaves a trail of death and decay in its wake. The only way to stop the Scathing is to retrieve the soul of the goddess Danua from within the fae realm. Maeve’s mother sends her off with one condition…return with the soul, or don’t return at all.
But Faeven is nothing like the fairytales she’s read; it’s full of ancient secrets and wild magic.
Determined to save her kingdom, Maeve has no choice but to rely upon Rowan, the one faerie she shouldn’t trust. But as her feelings for Rowan transcend from hardened resentment to blind passion, a more sinister threat lies in wait, one ready to destroy the fae and human worlds. Maeve must find a way to defeat it, or lose both realms to the rising darkness forever.