Angel's Demon. Angel Series, #5

Protect Little Mother, when the time comes ... Satan's archdemon, Asmodeus, is jealous of the relationship the earth-bound angel, Helena, has with his lord and master. He has devised a plan that will rid them of the harlot for good, one that requires stealth, loyalty and a sacrificial lamb. Gina's children are awakening, only to find that someone is methodically torturing and murdering anyone who has met her.
The self-proclaimed angel followers seek out Helena, who they're supposed to protect, each of them wondering what an immortal could possibly need protection from. They believe that in the end it is she who will protect them. When Helena learns of what's going on in the world around her, she tries to save as many lives as she can. Yet to stop the carnage she must come face-to-face with another earth-bound angel and defeat him - or die.