After the Cataclysm of the old generation of heroes and villians, Asgard was left in ruins, as the last phases of Ragnarok occured during the Cataclysm. Fortunantly, the cycle of death and rebirth began again for the Asgardians, and soon new Asgardian gods began to appear. One of the first to appear (the third one acually), Asura, along with the other emerging Asgardian Gods, started helping rebuild Asgard. However; as they rebuilt thier home, Asura began to worry about Ragnarok, which had caused thier demise before. His worries grew and grew untill it became a fear that gnawed at the back of his mind at all times. Slowly, his fear became insanity.
Soon, he became so insane that, in his mind, in order to prevent another Ragnarok, he must slay or contain every single being on the Earth. He started by destroying the newly built Asgard, and trying to slay his fellow Asgardians. However; he was fought off, and in the resulting explosion that occured from the collision of thier attacks, Asura was knocked out, and the other gods banished him to Earth.
On Earth, he had no idea on how to get back ot Asgard, as he had never left Asgard before. So, he decided to try to enact the other part of his plan: to contain all beings on Earth. However; he feared he would be killed if he tried to contain everyone all at once (He thought that on Earth his powers would be extremely weakened), so he sought a way to increase his powers. He had been searching for a few days when he met Dr. Krillian.
Dr. Krillian was a ex-military scientist, who had been kicked from the army for his dangerous experiments in weaponry and armor, which had on more than one occasion resulted in injury of soldiers. However; what had got him kicked was when he made what he called "the best armor ever created", the Aidoneus Fusion Armor IV. It had been the latest in a series of armors that, instead of using an external energy source, used the person's own vitality to power itself. However; all of the soldiers who tested to armor out died, as the armor required alot of power, and all of thier vitality wasnt enough to power it all the way. Even after he was kciked from the army he tried to get people who wold test out the armor. He was desperate.
So, when Asura met Dr. Krillian, and told him what he needed, the scientist offered to have Asura test out the armor. After a painful procedure, in which besides fusing the armor to Asura Dr. Krillian replaced his left eye with a targeting monocule, Asura, being an Asgardian, was able to power the armor fully without dying, much to Dr. Krillian's delight. Unfortunantly, it was such a strain on Asura's body that he fell into a coma. Dr. Krillian, afraid of repercussion, dropped Asura at a hospital and ran off.
Asura was in a coma for two weeks before he woke up. Sadly, he had amnesia, as it was the only way his body was able to stand the pain of the armor. He was sent to an orphanage (because they thought he was around ten, given his small size, youthful appearence, and speech and ), and soon adopted by a kind couple, who had felt sorry for what had happened to Asura.
For five years Asura lived with his adoptive parents in relative happiness. However; slowly, he started to regain his memory as it grew closer to the five anniversary of him being adopted. And on the day of when he was adopted, his memory came back, well, at least the memories of who he was did. For some reason, the memories of how to apply his powers at God-level strength have never come back. He has speculated that perhaps the other Asgardians had taken them away in case he ever went insane again, but no real answer has been found.
Anyways, once Asura remembered what he had done, he felt he was endangering his adoptive parents, for if he became insane again he might hurt them. So, the day after, he fled from his parents. Fortunantly, his Asgardian siblings, who saw he was truly sorry for what he had done, visited him, and convinced him to go back home and live a happy life. For three more years he lived happily with his adoptive parents who, though surprised at Asura's history, were completely supportive of him. Once he was eighteen, he decided to become a hero, in order to both protect his parents and the world as well. Secretly, he also wanted to do it so that if any threat that could result in Ragnarok appeared, he would be able to face it, with other heroes like himself.