We know, for certain, Godfrey was Marika's true first Elden Lord. But, even it being a common sense that Godwyn was their firstborn, there's actually no true evidence claiming he was born before the omen twins. Except that now, after one piece of dialogue in the DLC, can shows us this and a little more.

First things first. Messmer's dad still is the great mystery of Shadow of the Erdtree, and again I think this is on purpose, because considering Godfrey's evidence, the game wants to tell us he was already there when Messmer came to life. And I, personally, don't think neither Radagon or Godfrey are Messmer's father. I think, in a parallel to christian God, sacrificing his son for the sins, Marika birthed Messmer alone — and Melina, that's why they both share a vision of fire and are direct siblings.

According to Marika's own words: "[Radagon] Thou'rt yet to become me. Thou'rt yet to become a God. Let us be shattered, both. Mine other self."

Knowing that Marika comes from the Shaman Village, and that her people was "grafted" to each other in seek of the divine, he is a part she divested herself later in the story and Messmer was some sort of manifestation of this, but not necessarily a child of Radagon.

And I guess we can agree Messmer was born in the Lands of Shadow, because he knows very well the sins committed against his mother and her own modus operandi. He was there before her godhood and that's why Godfrey's offspring got the title of first demigods. And Godfrey, too, was there with Marika since the start.

I won't get into the timeline of events, because it's a mish-mash in game, and I don't have the tools to explore assets and files. But, one thing that the DLC gives importance is the act of a vow. And we know Marika and Godfrey did a vow.

Leaving his past of Hoarah Loux, Godfrey took Serosh upon his shoulders to ease his bloodlust and accept the role when he sworned a vow with Marika to become a Lord. Which can reminds us of the Secret Rite Scroll, found in Shadow Keep.

"A lord will usher in a god's return and the lord's soul will require a vessel."

We can argue here about the roles of vessel and soul, because it feels Godfrey became the vessel to the Beast Regent (some sort of lord), which recalls the amount of lion representation in Hornsent culture. So, I tend to think that Serosh was part of the "betrayal" in order to ascend Marika to godhood with Godfrey as her lord consort.

That's when I imagine Godwyn was born. After the vow, to consumate the idea of Godfrey being named a lord of the golden lineage once Marika achieved godhood and started the Age of Erdtree.

If we recall the Minor Erdtree, left to the matriarch at Shaman Village, it's said:

"Secret incantation of Marika Only the kindness of gold, without Order."

I can see this as Miquella divesting himself of St. Trina, his love. Marika bathed her home in gold, knowing there was nothing left to heal. Alongside with her braid, it was a departing gift, because she was leaving for a new era. She is leaving her kindness of gold, to built the order. Godwyn was her last act of kindness of gold before the Order. That's why we have so many evidences he, alongside Messmer, was her beloved child of gold.

Going deeper, once we advance the questline of Hornsent Grandam, she tell us a very specific dialogue line, that can show us why Godwyn was curseless when he was born, different from his siblings.

She spoke of a omen curse. Curse upon her children, each and all. Messmer as her own flesh and blood, which I believe, carried the grafting curse upon her people over and over again, being a vessel to an outer god "grafting" into Marika's child. But Godwyn was curseless, because he was conceived outside cursing eyes, in secret. He was born of a vow that, once came to common knowledge, was called "the seduction and the betrayal" in the eyes of the Hornsent.