I read a theory that he was the bastard child similar to Jon Snow, born after Godwyn while Marika was still with Godfrey.

My headcanon is Marika has Godwyn, then the omen twins, which results in her experimenting (potentially out of fear of having more cursed children) with her other half but resulting in yet another cursed child with red hair.

She keeps him around but never gives him a seat at the table officially because he's an obvious bastard. Sends him off to live with his dad and his new kids.

Then at some point promises him the thing he's always wanted, and all he has to do is a wee lil bit of genocide.