The Lethe Stone

The great battle for the survival of the Fae is over…but the war is not yet won. Though they have defeated their greatest enemy, evil has slipped through the veil into the human world, and a cursed bone sorcerer wields his deadly power against helpless mortals.
Tess O’Connor, the not-quite-mortal Bearer of the Iron Sword, must now strike a pact with the three Queens of Faeortalam to open a new Gate between the worlds. With a hand-picked band of warriors, she ventures back into the mortal world to search for a missing ulfdrengr, a Wild Court navigator, and a human who had fought alongside them at the Dark Keep, hoping to find them before the bone sorcerer.
In the Fae world, Queen Vell balances her alliance between the Seelie and Unseelie Courts. One of her Three has a vision of a bloody civil war, and she fights to hold back the tide of enmity between the Seelie and Unseelie while the Bearer is on her mission in another world. With their common enemy defeated, the High Queen must hold the tenuous peace between Queen Mab and Queen Titania or risk plunging Faeortalam into another deadly war that may be the end of them all.