When two demigods marry and have children ,they would be gods ( sort of ) because half god plus half god is full god and half human plus half human is full human.
So what would the child be ?
The child would be a legacy as they're descended from two gods.
A legacy
^ Yep.
If, for example, Percabeth had a child, the child would be quarter Athena and quarter Poseidon, because the godliness would dilute each generation.
25% god, 75% human aka legacy.
