Aqua was about to go take a bath with someone grabbed her from behind. Aqua thought it was Darkness at first, but realize that, although the woman looked strikingly similar to Darkness, her eyes indicated that she was not Darkness. Before Aqua would mutter a single word, the woman put her hand over Aqua's mouth as her entire began transforming into a thick purple smoke. Aqua tried to pull away, but it was already too late as the smoke rapidly entered her mouth.
As Aqua became possessed, she started to undergo a terrifying transformation. Not only did she became taller, stronger, and sexier than before, her power of purification was mixed together with Wiz's dark magic, becoming a new power to corrupt. Soon, a twisted smile emerged what was once Aqua.
As the possession was completed, the newly emerged being took a glance at her body. Her had Aqua's face, however, she was not Aqua, nor was she Darkness or Wiz. She had the memories and powers for all three girls, but she would only use them as tools to fulfil her own selfish desires. She is a newly formed dark goddess, born from Kazuma's lust within the those girls. She only had one mission, to corrupt all the girls around the world and make them become a part of Kazuma's harem.
And thus, Kazuma had unknowingly created a new powerful villainess. Although he is tempted to just let her create a huge group of female admirers for him, deep down he understood that he had to stop her. He and the rest of the girls would have to work together to undo this unholy combination of possession.
And, of course, he will take all the credits when he takes her down.