I am having trouble in the last scenario, "The Other Side". After I hav…

I am having trouble in the last scenario, "The Other Side". After I have completed a few quests as a knight, I am sent to the Bishop's church for a third time. The Bishop tells me something along the lines of: "Necromancers have created a black alter on Dead man's Island. You must find the Generation of Vipers and purify the alter with Holly Water." After defeating the Necromancers, I enter the alter. Nothing happens. I go back to the church and the quest is not complete. I am stumped. Anytips on how to beat this quest?