"Originally published in single magazine form in Shadowpact # 1-3, 5-8."--T.p. verso.
OBNB doesn't sell books, but you may be able to find a copy at one of these websites:
Shadowpact. The pentacle plot by Bill Willingham. ISBN 9781845765330. Published by DC Comics in 2007. Publication and catalogue information, links to buy online and reader comments.
