Were the demigods defeated, or did Porphyrion surrender?
The demouement is like when the climax ends, or in this case, when the battle ends.
The denouement in this story is when Porphyrion decides to come back another time. Why did the giant suddenly surrender? He surrendered because the demigods had freed Hera, and a god/goddess' power is infinitely...powerful. Porphyrion didn't want Hera to defeat him, so he left and decided to come back another time when such a powerful being was around!