Find the Altar of Hakkar in the Sunken Temple in Swamp of Sorrows.
Inside the temple, I found a circular room with six balconies, each one with a serpent statue on the edge. Curious, I pushed one of them to see what would happen, and it was a trap! I nearly fell to my death from the explosion. Laying there on the ledge, I looked down and noticed that there was another statue at the base of the room; an altar of sorts... Hakkar, I think it was called.
I think that this altar is the secret -- you must bring this token to the altar in the Sunken Temple.