Having possessed so many girls at once, including the two goddesses, the dark goddess was at its most powerful state than ever. Even Kazuma's best effort was no long sufficient to defeat her. However, Kazuma knew that a direct confrontation would not work well for him, therefore he had a contingency plan in place. After many encounters, he had decyphered that this dark goddess persona originated from the built-up lusts from him constantly possessing the girls, thus it was technically a part of his consciousness that got tainted by the different personalities of the possessed girls. Thus, when he was brought within arm's length to the dark goddess, who was in control of a corrupted adult Iris, Kazuma cast the possession spell one last time, possessing the dark goddess itself. Once possessed, the dark goddess's consciousness began rapidly breaking down as it was reabsorbed back into Kazuma's own consciousness, allowing Kazuma himself to assume control.
After having some fun with the body of the heavily transformed Iris, Kazuma decided to use the power of the dark goddess to return everything back to normal...
Well, almost back to normal. Taking advantage of this oppotunity, Kazuma implanted complete copies of his personality into the minds of every possessed girl and goddess, effectively turning them into "himself". Knowing that he would never betray himself, this not only prevented the rise of another dark goddess, but also allow Kazuma to call any of them to his side at will.
The is still a world left to be saved, but that is story for another day.