Veil of Embers: The Threadfire series

Eadartha is rotting from the inside out. Sorcha is a druid ranger sworn to guard the Veil, the shifting boundary that protects her realm from the creatures that lurk beyond it. When a cloaked figure whispers her name, the Veil begins to tear. Monsters slip through the cracks, spirits stalk the woods, ancient gods stir awake, and Sorcha's runes flare with a power she never wanted. The Druid Council offers no answers. The forests offer only warnings. And the Dark Druid's shadow grows stronger...