It could also be a big part of why Renalla is so torn up and despondent. It wasn’t just Radagon leaving her, but also from a lost child. Double tap.
Considering that the Rune of the Unborn was removed from the Elden Ring BEFORE the Shattering, and that Radagon IS Marika, I suppose it’s an option. Perhaps they tried to use the rune to try and save the child, but it didn’t work.
I’ve also heard theories that the wedding was a scheme to weaken Renalla and the Carians the entire time, that Radagon poisoned them from within and then returned as the second Elden Lord to seal the deal. It could be both. Perhaps the Knights of the Cuckoo aren’t just a fun name, but a tell of what Radagon did. He replaced Renalla’s children and attention towards her own family to an impostor. And it worked. Caria fell, Renalla became a shell of a woman, and the only member left that is doing reasonably okay is Ranni. If you call being a doll and committing heresy and blasphemy okay. If that’s fine, than I suppose Rykard’s pretty based as well.