The Wulfen book has the same issue with the Death Pack formation. It is a core choice. However, it is not listed in the "Decurion" map in the Wulfen Supplement. But, the Wulfen book says that it IS a supplement meant to add to or replace entries in Codex SW. That is why I can't understand those that still try to use the Iron Priest as an Elite. It states very clearly in the codex that it REPLACES entries in Codex SW.