When the Demon of Song developed a taste for human flesh, it was contained within the Shrine of Amana. A line of priestesses was put in place to look after the Shrine and appease the demon by sacred rituals, but they have since died off or gone hollow and over time the tradition was lost and the Demon set free. The Demon gained knowledge and learned to lure humans into its lair. It is called the Demon of Song because it sings with a sonorous voice to lure people close, so that it may devour them. Its song is that of the Milfanito. Within close proximity to its lair we find a Milfanito who appears to be hollow, and cannot sing. It may be that the Demon of Song somehow stole the voice of this Milfanito.
What the sacred rituals were that the priestesses performed is unknown. It may be that they sang to the Demon like the line of priestesses in Shulva, the Sunken City. This may be the case because the Archdrake Sect, who the now hollow Priestesses of Amana fight with almost certainly originated in Shulva and may have therefore passed on the rituals of their priestess line. Also, the Demon appears to have been contained prior to the Player entering the Shrine of Amana and not gaining in knowledge. Why would it not learn to sing if it had that capacity and if it was conscious of its surroundings? However, it may be that the Demon was simply confined and had no need to sing to lure humans in because none would be lured.
The spotted whip that can be created from the Demon of Song's soul is coated in poison. The Demon of Song itself does not deal any poison damage, but it is contained within the body of a frog or toad, which in reality, can be known for their poisonous qualities.
The Demon drops the Key to the Embedded. How it came to possess this key is unknown. However, it is interesting to note the similarities between the Demon of Song and The Embedded. Some examples are:
1. The Demon of Song’s soul description says that it developed a taste for human flesh, while the Key to the Embedded says that the Embedded realized that he could never resist the temptation of the flesh. When the Embedded realized he couldn’t resist, he bound himself in chains and when it was realized the Demon of Song had a taste for human flesh, it was contained in the shrine.
2. Furthermore, The Embedded bares all for anyone to see, hiding only his face behind the mask, the Demon of Song hides within the body of a toad, and is only a face, arms and hands.
3. The putrid and pallid skin colour of both the Demon of Song and The Embedded is also coincidental.
4. The Demon of Song has captured the voice of a Milfanito which wastes away in a hut just outside its lair, and The Embedded is the gatekeeper to a cage that houses a healthy Milfanito.
These may simply be dismissed as coincidences, but their relationship is cemented by the fact that the Demon of Song drops the Key to the Embedded when defeated.