Is it possible to have some sort of evolution chain for the undead so that they can gain the ability to harm things normally immune to negative energy? Being a god of negative energy is only fun if you aren't totally shut down by a single 4th level spell. Perhaps something along the lines of you can deal 1/4 of normal damage with it per mutation, improving by that each time maybe?
EDIT: Maybe another option for Teratomorphism II and IV?