Odamori Tooru and his entire class died on a field trip, but Tooru was reincarnated in another fantasy world. Given the title of Slavemancer by a strange man who sent him there, he has the ability to control people’s minds and turn them into his willing slaves. Having too much fun with his abilities leads the kingdom to send a princess knight to subjugate him who turns out to be another of his classmates who was also reincarnated. He makes quick work of her and adds her to his harem, although she begrudgingly fights him to the best of her ability. Now Tooru has a small army of beautiful girls as his slaves, and it’s anyone’s guess as to what he plans to do next.

Satou has been in love with his childhood friend Hime for 10 years, but she suddenly tells him that she’s from another world and has to return to it. Before the portal she left through closes, he follows her and discovers she was going off to become the wife of the Ring King who will save the world. Not willing to give Hime up to someone else, Satou becomes the Ring King himself and vows to collect the other rings from the other princesses and defeat the Abyssal Lord. Thus, Satou begins a long journey to amass a harem that will give him the power he needs to build a peaceful world to finally share with Hime. The other Ring Princesses want to have him as well.

Tada Youta was pulled out of his classroom window by a mysterious figure to what he thought was certain death. However, he awoke to find himself in a strange world with dragons soaring through the sky, and an angry sword-bearing girl trying to attack him. He is told by a god of the world that he is now in a place that hasn’t seen a human male for countless centuries and that he is the only one. The girl, Misaki, begins to convulse and show signs of serious arousal, and the only way to put an end to her shameful display is for him to mate with her.

Soon afterward, Youta is meeting and mating with multiple girls as he learns a dark secret of this world. There is no childbirth and the young die young. Only by mating with the girls he encounters and finding the source of the world’s imbalance can he hope to save it and find a way to return to his. We’ve known manga to have some pretty big harems in the past, but Youta has an entire planet of women all to himself. You don’t get much larger scale than that.