Continue your journey with Prima's Dark Souls 2 Guide and Walkthrough!
Reward: Demon of Song Soul, Key to the Embedded
The Demon of Song is essentially a frog with a demon in its mouth. When its mouth is closed (which is about half the fight), the Demon cannot be damaged. The Demon's main attacks consist of two-hit combos using its arms, and a jumping attack that looks more like a falling attack.
Unlike most other boss battles, stay back far enough to avoid the Demon's arm-based attacks. After its various two-hit combos, move in and attack it as much as possible until the mouth closes. Stay close until the Demon appears again, then move back out of range. You need to remain close while its mouth is closed to ensure you have time to run under it if the falling attack begins.
As soon as the Demon stands up on its hind legs, run under it to avoid the falling attack. At the completion of the attack, head back around to its head and continue with the original strategy. It shouldn't take much to bring the Demon of Song down, so long as you're patient.
Kill the Demon, then head into the cave and down the next path. Continue through the structure ahead, then proceed to the far left corner near the stairs to find a chest that holds 20 Magic Arrows. Head up the stairs and into the next room to find a bonfire. Light the bonfire, then head back outside and through the door on the far right. Take the lift inside down to the lower floor, then head down the hallway into the Undead Crypt.