This achievement is unlocked on the 3rd mission of the second Slavs campaign, "Boleslav I".
After taking the "B" village on the left side of the map, you will get a side objective which you simply have to follow to it's conclusion. It's down in the southwest corner of the map. There will be a couple of cutscenes with a pagan priest. The achievement will pop as soon as the last cutscene with him ends.