After defeating Demon of Song leave the arena through the exit on the left of the entrance to the fight.
Turn left and you will see a semi-submerged path.
Follow the path until you reach a tower. The bonfire is inside.
Up the stairs there is a door. You can only enter if you are hollow, have committed no sins and have not killed any of the Milfanito
At the end of this area is a shrine which you can pray to in order to revive yourself to human if you have no human effigies in your inventory. Dropping your effigies before praying does not fulfill the requirements of the shrine, so you need to have used up all of them in order for the shrine to work. For more information see Shrine of Amana