Disclaimer: I don't own harry potter
Failsafe
Tom Riddle was not stupid his horcruxes may have taken all his reasoning and made Voldemort but, what people forget is before Voldemort there was Tom Riddle. Tom Riddle's one drawback may have been that he was afraid of death but, it was a strength as well. Before Voldemort there was Tom Riddle and he would not pin his chance of death on just one thing. Voldemort was insane and lost his mind he believed that he was invincible. Tom Riddle however, still remembered those days in the bomb shelter cowering from muggle bombs when he had to rely on luck to survive. So, while tom riddle created horcruxes he also had a failsafe to ensure his life.
So on a battlefield, when two spells 'Expelliarmus' and 'Avada Kedavra' collided at the battle of Hogwarts, everyone remembers how it rebounded and killed Voldemort but, no one remembers that color that happened when both spells one representing good and one representing evil mixed. It created something neutral but, it was gone before anyone saw.
This story begins fifty-six years earlier when a sixteen-year-old Tom Riddle may have been arrogant but, he was not foolish and he knew that he was far from invincible. So, at midnight exactly in a chamber of Salazar Slytherin he started a ritual that would make him immortal however, he was never able to finish it. This ritual was his failsafe, only to be enacted if his horcruxes did not work.
This ritual made the most compatibly powerful person that his magic had mixed with conceive a child with him right before he died. Whoever was dominate in that relationship would be the sire and whoever was submissive would be the bearer. As that child was conceived he would be called back before he crossed the border into death as the father, the child, and it's 'mother' would all become immortal. This ritual that should have otherwise failed because, mixing magic usually refers to sex and all the people both Tom Riddle or Voldemort slept with were dead.
Lady Luck was on his side however, and when both his and Harry Potter's spells clashed it made both of their inherent magic mix. While if they were muggle this would not have made a difference Tom Riddle was lucky males in the wizarding world could get pregnant. This made quite the difference and the ritual was completed Tom Riddle became pregnant with Harry Potter's child and that made all the difference. This being they were bonded right there for all eternity and a brilliant white light spread out. It dimmed too quickly for anyone to notice the binds on both Tom Riddle and Harry potter hands however, the wizarding world would soon know and regret with all they had.
This was for another time as right now all the wizarding world was doing was celebrating and hugging so, no one had time to notice their savior standing there with and empty glazed over look in his eye.
Harry was feeling weird like he was missing something while everyone was busy celebrating he felt like a piece of his heart was not there. He quickly got over it though and started cheering with everyone at that night he got drunk and forgot all about the feeling for that night.
Edited March 20th, 2018