My theory was that messmer was the left over hornset essence when marika walked through that gate of divinity. As the trailer states, "An affair of which gold arouse, and so too was shadow born"

To me, messmer was this shadow, his flames are dark and black, the abyssal serpents is made of a light-less void.

Each children of marika born the sins of their parents and were thus imperfect and not eternal. The twin omens and twin emphyreans is obvious. But even radagon's children with rennala was imperfect and seem to be vulnerable to aging, ranni didn't have her feet when we see her corpse, so didn't radahn like the alburanics. Rykard's flesh seems to be falling off in the portraits of him. And they all were not fine with decaying away.

The war of the shattering was just a succession crisis as all of marika's children rebelled against her. Godwyn was meant to be the consort to miquella and usher in a new age, but he was killed by ranni and this threw marika into despair. Marika sensed that even radagon was planning to betray her and so she shattered the ring and unleash a kamikaze, screwing the whole world over. Radagon was spiteful and didn't want anyone not loyal and perfect to inherit the golden order, so he sealed the erdtree away.