The Kane Chronicles : The Red Pyramid Book Cover Explanation?

I was just wondering cause i just finished reading the book. The cover is bothering me.I know that the building is obviously the mansion on the warehouse. and that Sadie's casting spells and that Carter's obviously ready to battle. Also there's a giant snake in the mansion. That part bothers me. Why is there a giant snake at the mansion? I don't remember that part. At all. Is that snake supposed to be Apophis? I don't remember there being a part in the book that's like that