If only they could behead the old Demon King and end the quest once and for all, however...

Before they knew it, all the brave warriors found themselves trapped inside a wall of tentacles.

Under the captivity of the Demon King, the hero was transformed and able to become pregnant while being trained by the demonic tentacles.
Then, the Demon King ejaculated a large amount of magical semen into the hero's body, making him pregnant.

It turns out that the hero will give birth to the Demon King's new vessel.
With this rejuvenated body, the reborn Demon King xxx the hero again, thus impregnating him with more demon.

This story revolves around the hero becoming the Demon King's mother and wife.