After you've beaten the boss in the church...
Go back to Cyseal and talk to Selenia. She is between Esmeralda's shop and the cook's house.
http://divinity.wikia.com/wiki/Divinity:_Original_Sin_Cyseal - She's between M and J, sitting at a table by 3 red tents. Achievement will unlock after you talk to her about the undead problem.