Shrike Killer (The Possession Trilogy)

A possessed child calls for the direwitch Lienne to investigate deadly bird attacks against the tiny village of Cupt. The entity responsible, Yog, calls himself a God. Based on his powers, Lienne can't be sure he's wrong.

Seeking to overcome an enemy that laughs off her most formidable efforts to destroy him, she and her old friend James team up with a young woman from Earth who hates her life so much she's already tried to leave twice. They find another ally in the vampire Sigrun, who Lienne is drawn to even in a timeline where Sigrun has no recollection of their previous encounters.

Their efforts to defeat Yog lead them to a weapon with the power to vanquish World Heart, the hive mind that was previously the greatest threat to humanity on the planet Elysion. It proves useless against the self-styled god. World Heart strikes such fear into Elysion's people that many of them will accept alien DNA as a defense against it, causing their own government to declare them no longer human. Yet this ancient enemy could prove to be Lienne's only effective weapon against Yog.