Cadmus wasn’t sure he should be stealing the dragon’s dental work, especially if he was already on Ares’s naughty list, but he did as Athena commanded. When he was done with his tooth farming, a bunch of super-elite skeleton warriors sprang from the ground, and these became the first soldiers in the new Theban army. Cadmus built his city. For a while, everything was copacetic. The gods even granted him a minor goddess for his wife—Harmonia, who was a daughter of Aphrodite and Ares. Harmonia became mortal to share her life with Cadmus, which was a pretty big honor.
"Ares, The Manly Man's Manly Man", Percy Jackson's Greek Gods, Rick Riordan
I realize Mr. Riordan's books target a younger audience, and this may seem like a minor detail in the grand scheme of making the myths accessible to youngsters. However, I feel it's a rather important one as Gods (and other immortals) turning into mortals is not something that happens in Greek mythology. The only story I know of that comes close is Castor and Pollux, yet not close enough, as the twins alternate between mortality and immortality. As for Harmonia, I checked several ancient versions of the myth, and she remains immortal in all.
Has Rick Riordan ever commented on how accurate his portrayals are intended to be? And given that several of the stories he presents may differ significantly from source to source, has he ever provided a list of sources he used in his research of the myths?
This was inspired by a Mythology Stack Exchange question: Are there any Greek myths of a god permanently turning into a mortal as punishment?