War of the Fae (Book 8, Time Slipping)

Jayne and her crew of fae friends are off to Uamh An Ard-Achaidh, the High Pasture Cave in Torrin on the Isle of Skye, to fulfill her destiny as the companion to the portal guardian of the Underworld. But on the way there, weird things start to happen, and she's forced to reckon with at least one behind-the-scenes player bent on keeping her from getting to the portal on time.