So crosses don't do anything against your kind?" Sean asked. "No," Arland said. "There is no mystical force repelling us." "Then why?" "We're forbidden to kill a creature in a moment of prayer or invocation of their deity. Well, we can, technically, but you have to do penance and purify yourself and nobody wants to spend weeks praying and bathing themselves in the sacred cave springs. The water's only a fraction warmer than ice. When one of you holds up a cross, it's difficult to determine whether you're praying, invoking, or just waving it around. So the sane strategy is to back away.