The most common Doms2 undead-damaging sity seemed to be the nature site 'sacred grove' (or something like that). While it does sound 'holy' it is found with the nature skill... the holy sites that damaged undead seemed to be rare.
Have things changed in Doms3? If not then nature's definitely worth trying.
I /think/ that undead damaging sites actually deal fire damage, so resistance to that protects you... but I could be wrong...
Anyway, I'm sure that the effects can kill a 6hp creature. If your pretender's outside his dominion then move him fast! I'm sure that the effects can kill a 6hp creature
Immortality also only works in friendly dominion, which is an even more important reason than the HP bonus to wait until the province is firmly in the white.