"Holy force native to the province" killing undead

[Szass_Tam]

I've come across a province in which I get the message every turn that "Your undead troops in ProvinceName are being killed by a holy force native to this province". I know there's a site there, but I'd like to know what it is and what level/type so I can search it out. A H3 priest already failed to turn up anything, which is odd because I assumed it was a holy site. Is it from another path or do I need to find a H4?