Bring Him Back, Jack

Once every century, a shadow walker is born. Ivan Charna is that shadow walker, and he and his sister Raina are on the run from wizards who want to sacrifice their blood in some crazy rite. They never stay long in one place, and they do their best not to stand out. When Raina loses a bet, Ivan is forced to work off her debt, and they have to stay put. As long as no wizard figures out what they are...