Darktide by Christine Pope

No turning back. No backing down. And no surrender...
When Angela McAllister and Connor Wilcox are asked to help find out who-or what-killed a de la Paz clan private investigator, Angela agrees with only a slight hesitation. After all, with Connor at her side, she and her unborn child should be safe.
They quickly learn the entity haunting the Tucson crime scene is stronger than anyone anticipated, with uncanny powers they can barely fend off, much less identify. Only Levi McAllister, with his otherworldly origins, has a chance of figuring out what they're up against...but he insists on going alone, and only after Angela and Connor's twins are hidden away.
But when Levi vanishes, and news comes to light that a pair of old enemies have died simultaneously, Angela and Connor realize their clans are being set up for destruction.
The days of choosing whose unique powers best fit a given situation are over. Only by joining forces and combining powers-and learning to control dangerous new ones-can they defeat their deadliest enemy.
Once and for all.
Darktide is the final novel in the Witches of Cleopatra Hill series. The series is complete and should be read in this order:
DARKANGEL
DARKNIGHT
DARKMOON
SYMPATHETIC MAGIC
A CLEOPATRA HILL CHRISTMAS
PROTECTOR
SPELLBOUND
THE ARRANGEMENT
IMPRACTICAL MAGIC
STRANGE MAGIC
DEFENDER
BAD BLOOD
DEEP MAGIC
DARKTIDE