They didnt even need to make Nald'thal, they could have just quelled that rage with some crazy hate sex that slowly turns into a yearning but neither wants to admit it first so Halone keeps fighting to get Nophica's attention and then they start kissing and then they realize-
In the Eorzean creation myth, Halone developed the art of killing. Seeing her "children" die, Nophica flew into a rage. To quell her, Oschon created Nald'thal from magma--to oversee those who met their death and provided peace to those who passed.