This unusual fight stars a frog with a demon tucked away in its mouth. Whenever the mouth is closed - approximately 50 percent of the fight - the creatures can't be damaged, so don't even try. As for its attacks, expect a flurry of two-hit combos from the creature's arms, and a leaping attack that actually looks like a falling attack.
How to kill the Demon of Song
The first important thing to do is keep as much range as you can between your character and the boss. That'll keep you away from the dangerous arm attacks. Whenever it's finished one of its two-hit combos, move in and swipe away until the boss closes its mouth. Stick close to the enemy until the demon shows up again, then put distance between you once again. The reason for sticking close while the mouth is closed is to allow you to get out of the way as soon as the falling attack starts.
When you see the creature get up on its hind legs, run beneath it as quickly as possible to avoid the incoming damage. When it's over, get back around to its head and repeat the previous strategy. Keep this up and it shouldn't be long before the boss goes down - just be patient and play defensively.
After you've killed the Demon of Song, go into the cave and then make your way along the path. Keep heading forwards, then head over to the far-left corner when you see the stairs. You'll be able to grab 20 Magic Arrows from the chest that's tucked away here.
Now make your way upstairs and enter the next room along to light a new bonfire. Now head outside and go through the door over on your right. Ride the lift all the way down to the ground floor, then walk along the hallway until you come to the next region of the game, the Undead Crypt.