We learn from the end of her quest she is, in blunt terms, the product of an occult and implicitly torturous/painful sex ritual that creates the man-serpents. Tanith is her adoptive mother, but her birth-mother is pointed to be Daedicar, who is only characterized by their participation in the aformentioned ritual, which paints it as pretty extreme since it involved skin-flaying, and it's generally accepted that her birth father is Rykard himself, or what's left of him. (Given all the man-serpents guarding the way to his chamber and how they're the result of his intentional "experimentation" it stands to reason that it would be no different for Zorayas. )

It just hit me though... Rykard is a demigod. Sibling to Ranni and Radahn, son of Rennala, and easily the demigod that sticks out the most between them given his whole god-devouring serpent thing going on. So... What's the deal there?

The only things I can immediately consider as to why it isn't like this (outside of the meta reason of it being a little out of place for an entire area to be filled with demigod snake people, or it was just overlooked in the writing) is the consideration of the god-devouring serpent as a separate entity.

While this is technically confirmed in the boss fight, we see they're still one comprehensive entity regardless when the serpent wakes Rykard up when it starts to lose to us, which shows us he's still in there and present. We're given the impression from the ghost that Rykard's still alive but after giving himself to the god devouring serpent, his noble ambitions were corrupted in to mere greed. Tanith clearly doesn't even differentiate between them with the way she talks about him.

Deadicar's Woe makes it sound like the result of something that shouldn't have been able to happen, and not just cuz it's gross, because it required a vague but sinister ritual to work, and I feel like it's something that wouldn't work without Rykard being able to bend the rules being a demigod and all, in fact Man Serpent ashes state this explicitly;

Spirit of a deformed man-serpent that wields a whip of magma. It is said that long ago, the elder serpent that dwelled on Mt. Gelmir devoured a demigod, and the birth of the man-serpents followed

This suggests not only that it was after the GDS and Rykard became one, but the fact that it happens afterwards implies it was Rykard's idea or at the very least, not possible without him. since the GDS and Rykard are one being, though separate entities, that means that Rya and the man-serpent race in general are technically of divine blood since Rykard is the father to all of them.