Thanks Shovah, I've had a look to Edi's dB and you're right, Mound Fiends (Id 439) are Reanimators, D3 Mages and H2 priests, all in one!
Most of my previous comments were based upon Sunray's "units_dom3.xls", where Mound Fiends have the following incorrect description:
can reanimate but does not know unholy spells. For that reason I thought that there was no unholy priest summonings available for all Nations.