Too weak to fight without blood

Undeterred, the spear-wielding vampire came on, and sank its fangs into Thomas's forearm. Thomas shoved the vamp back, but it held firm. Thomas switched tactics, abruptly lifting the vampire up and clear of the ground, and then rolled the sword's blade around its belly, splitting it open in a welter of gore. The vampire fell to the ground, a sound bubbling up from his throat that was one part fury and one part agony.
"Their bellies!" Thomas shouted. "Without the blood they're too weak to fight!"
Michael caught a descending machete's blade on the metal guard around his forearm, and whipped one of his knives across the belly of the vamp who held it. Blood splattered out of the vamp, and it went down in convulsions. "I know," Michael snapped back, flashing Thomas an irritated look.


Grave Peril, Page 317-318