My Space-Time System - Chapter 4: The magical spring.

To correct the mistakes of his past, a young man is forced to narrow the space between science and witchcraft, as he ventures on a quest to save his world from complete destruction.

I do not own this imageHe was a swordsman beyond compare—cold, calculating, and relentless. A man who chased power not for glory, but to break the fate forced upon him. Betrayed in his final battle, he died with his blade in hand... only to awaken in a world not his own.Now reborn in the world of High School DxD, Nero carries with him a mysterious System that allows him to defy logic:— Steal and evolve Sacred Gears— Accept world-altering quests— Access a store that sells power itself— And shatter the limits of every universe he entersWith the power of his Void-Forged Blade and a will forged in war, Nero walks among Devils, Angels, and Fallen—but he kneels to none.As he joins Kuoh Academy, crosses paths with Rias Gremory and her peerage, and uncovers the secrets of the multiverse, one thing becomes clear:This world, and every world after it, will tremble beneath his blade.

Lu Ran had long since known that his path in life differed from others when he awakened a special ability one day. Sure enough a mysterious black card appeared, giving him the chance to become a beastmaster. With his loyal husky by his side, he sets out on his path to greatness.

18 years have passed since I was born as Allen, the eldest son of an earl in this world, while still retaining my memories of the present world. This world is peaceful and a stable society. The only extraordinary thing is that I was reincarnated into a world that is lenient on the existence of concubines. I selected a silver-haired maid as a sex slave and second wife candidate from among 10 candidates after a strict fellatio practical exam. I then lived an erotic life at home and at school as an attendant, but my reincarnated life is so fulfilling that I'm worried it will explode, as my busty childhood friend, a noble girl who was unsuccessful, ran for the third wife candidate, and my stepmother, who was my virgin partner, was sold off.