The Altar of Dagon (aka Dagon's altar) is on Black Tern Island. It is accessible by boat from the Lakeside. It is served by an acolyte of Dagon and is where sacrifices to the are brought, and so is the location of Geralt's duel with Dagon. After the duel, it is also where the Lady of the Lake speaks to Geralt about Destiny, knights him and gives him Aerondight.