Blood (The Unfinished Song, #6)

From Amazon:
Dindi and Umbral have an uneasy truce, forced to work together to defeat a greater enemy: the Bone Whistler. The Bone Whistler’s scheme to sacrifice humanity and resurrect the Aelfae will culminate during an eclipse on the spring equinox…in three days.

Their fragile alliance may not withstand the terrors they face. Dindi hides as a clown, but even disguised, her dancing draws the eye of the Bone Whistler himself. She will have to defy him alone, for Umbral has his own troubles.
Finnadro, who has hunted Umbral for a year, finally catches up with him… determined to punish Umbral for all his black deeds.

Life and death, spring and autumn, human and faery, are all reeling out of balance, and the next three days will determine the fate of all Faearth.