The Last Faith

Note: For this achievement, you need to have both of Esk's eyes in your inventory. Refer to Esk's Secret (15G) for more information.

After you beat Lord Patryk Laddak in the Junas Ministry, a warp will open up in the center of the Oxnevylle Manor. This warp will take you to Wonder of Illygarth. This is just one small area with a save room, an empty room, and the final boss room. 

Make your way to the east to the boss room. When you get there you will have a discussion with some of Eryk's old friends. After some talking you get two options; 'Accept' or 'Refuse'. Select Refuse and you will have to fight Caterina and Annabella. This fight has two phases so be ready. 

After this fight, if you have the two eyes, you will get a cutscene and then you will have to fight and kill Medeya. Once you kill Medeya, the ending will begin and this achievement will unlock. If you end up dying to Medeya thankfully you will restart at this fight if you die. If your low on healing items after fighting Caterina and Annabella you can die if you want and get your supplies replenished.