There is very little that I have found on this topic, like Pharros the Vagabond and Alva the Wayfarer.
All that is clear about the Embedded is the description of the Key to the Embedded:
Sword that opens the Embedded's door. Shaped like a weapon, but is in fact a key. Plunge the key into the Embedded to bring a rhapsodic end to his fate.
The once-human Embedded, realizing that he could never resist the temptation of the flesh, bound himself eternally with chains. Since then, he has awaited the day that somebody will find this key, and bury it within his bosom.
The whole thing is just disturbing, and just a touch Freudian. However nothing about it sheds any light on the matter. There are theories that he is Alva, and other theories that he is Pharros (though personally I think the theory about him being part of the Rotten makes more sense).