Source: Libris Mortis
You have learned to use the necromantic energy that powers your undead form to overcome the inability of undead creatures to wild shape. You can assume the form of an undead, rotten creature with the use of your wild shape ability.
Prerequisite: Undead type, wild shape class feature.
Benefit: You can use your wild shape ability even though you are undead. The ability functions just as if you were a living creature using the ability, with the following exceptions.
You do not gain a Constitution score in the new form, and you retain all the immunities of the undead type while in your new form. The form you assume looks half-decayed, with missing patches of fur and rotted, worm-eaten flesh. While you are in this form, the rank odor of death hangs around you.
Normal: Since it is based on the polymorph spell, wild shape works only on living creatures.