At the dawn of the universe, the Infinity Stones gathered. They came to an understanding that should anyone hunt them down in the hopes to destroy humanity, then the Soul Stone would vanish and become one with a pure soul. That way the seeker could not successfully obtain the Soul Stone as the destruction of the stones being would be unsuccessful. In fact, there would be no actual way to obtain the Soul Stone. Thus putting a hole in the seeker's plans.
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In 965AD, the Space Stone was left on Earth by the Asgardian's and later found by a madman, where it eventually transported Schmidt to Vormir, the home of the Soul Stone. Knowing Red Skull would never be able to access the power of the Soul Stone, yet still worried as the Tesseract returned to Earth only to be found once again by a man named Howard Stark, Soul decided it would be best to begin thinking about the plan the Stones had come up with at the beginning of the universe. Being the Soul Stone, it had the ability to see every single soul, so it began to search patiently for the perfect soul to host the stone. It's gaze kept going back to one Howard Stark. Then it flickered to the man's wife, Maria Stark. It was the year 1969, the Stark's were expecting. Soul was focused on the pure soul it could see growing inside the woman. The sentient stone smiled to itself and during the birth of the Stark heir, Soul transferred itself into the newborn where it would protect the child forever and unknown to the child, he would protect the stone.
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The number of times Tony should've died and he didn't. Well, how long have you got? 'Cause the list is long. Having your chest cut open and having your insides being moved out without anaesthetic or well anything, is a sure good way to easily die. Add in the cluster of shrapnel headed straight for his heart and only stopped by at first a magnet and rusty car battery. Don't forget about the time he was temporarily paralysed and had the arc reactor ripped out by his godfather -Tony shouldn't have been able to survive that let alone be able to move whilst under the paralysis. Oh also being electrocuted on top of the building where Stane died and he didn't? Yeah, that was unusual. Then he had the joys of being poisoned by the very thing keeping him alive whilst fighting a madman and an incompetent one (yeah, looking at you Hammer). Following those joyous events he was strangled by a God and then thrown from a window -seriously, that should've done some damage to him with the force, then being suffocated in space -terrifying and deadly, can't forget about the human-killing height he fell from -what were the odds that the Hulk just so happened to catch him? Of course, having his house blown up and him almost drowning and all that Mandarin debacle. Definitely can't forget about the time two Super Soldiers kicked the ever-loving shit out of him -Vibranium to the chest despite protection doesn't mean shit. Let's not forget him being left without communication in the cold, bloody, bruised and broken. See Tony Stark was special that way. He just wouldn't stay down…or dead.
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After his wakeup in Afghanistan, Tony became a very soulful guy. He doesn't fear the Hulk, he respects him and Bruce, telling the latter to stop running from his demon(s). He understands them. Then he was willing to sacrifice his life for a world that never cared for him. Not really. He does his best by people. By the world. By Pepper. His biggest fear is seeing all of his friends dead. He goes as far as to help create an AI that was supposed to protect Earth from the threat he knew was imminent. But, Soul's sibling Mind alongside the witch had to mess that up. (Let's not ignore the fact that Tony mourned the loss of his son, his AI, his JARVIS). He approved of the Accords (yes they needed editing) because they needed to work together to help lower the casualties and (what a surprise) to help protect everyone. He tries to talk to Steve Rogers, but the man would just have none of it. Tony continues to see the good in the man until well Siberia. But, even after the dreaded fight, Tony isn't consumed by hate and revenge like most. He knows that Team Cap is in Wakanda. He doesn't care though. At the end of the day, he knows despite everything that they can't be jailed as they will be needed in the endgame. In the fight against the monster that he has known about since 2012. The list continues but you get the gist.
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Our story starts where the end is expected. The year 2018. The Avengers are separated. No one expected it. Except for Tony Stark, who not only knew about Thanos, but he also knew something deep inside him had known since the beginning of his life.