Why was he set on drinking her blood? It's not as if he was hungry and if he was, she would have known about it. But here he was, still joking, still answering her questions. He didn't seem to be truly depraved of food to the point of starvation. He had also said it himself that he likes drinking animal's blood better and it didn't seem like he wanted to kill her. If he would, he would have drank her blood with or without her permission.
But Cassius was patient, watching her with his crimson eyes that did feel thirsty for a long while as she made up her mind.
Wouldn't it be painful? But why is it that she didn't only fear the pain? Rather she felt like she fear something else...
"If," she started, "If you drink my blood, you will grant me one wish?"
Cassius didn't plan on promising it but he didn't see how that was a problem. This is Arabella after all and he nodded with a smile.
