The Raw Material

He creates fiction.
His first novel concerns a young woman, Myrrha,
who falls in love with her father (a king).
When his queen (her mother) is away on a trip
they have a nine-night affair during which
he never-for-one-moment suspects
he is sleeping with his daughter.
(Besides, the nurse arranged the whole thing.)
When he finds out, naturally, he tries to kill Myrrha.
With a sword. She flees, pregnant,
turns into a tree, and gives birth to Adonis.
The king is left with a sour-lipped queen,
a tree-daughter, a son/grandson,
but a good conscience.