In Twilight's Shadow — Patti O'Shea

Something Wicked...
Maia Frasier thought she'd escaped the world of troubleshooters and demons when she'd ceded her magical powers years earlier. Now a wounded enforcer has turned up in her home, claiming Maia's sister is in danger.

Creed Blackwood needs Maia's help to hunt the demon he's after, especially since he's hiding the fact that his powers are becoming erratic. And then there are the blackouts...

Maia soon begins to fear Creed has fallen to the dark forces himself, and with his strong magic, that makes him as dangerous to her and her sister as any demon could be.