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The Girl isn't the only offspring Nilvan has.
For all we know, she isn't the only result of Nilvan's attempts at prolonging her existence via reproducing, and it's not exactly remarkable for a deity like figure to have numerous children. Nilvan could've tried mating with other men who she sensed were special or had a larger part to play, hoping their child would ascend into something greater. Heck, maybe she even tried other means of having children as this
concept art of the New Gods has beta Nilvan cradling what looks like a Demon Child. Perhaps in one of her attempts to bear a child worthy of carrying her legacy and also ushering in a new age, she did the "Use Demon Seed" spell and had her own demon son. Only, for numerous reasons, the resulting demon child didn't carry the same potential a human one would have. So she abandoned it in Ma'habre and her son eventually grew up there all alone until he wasted away, and after an unknown period of time, you find his corpse in the old passage.