Legion
There are many ways to describe Jefferson Jackson and Martin Stein. Friends, soulmates, two sides of the same coin. And, not very often but only later, one mind trapped in two bodies.
There weren't many similarities between the two. Of course, they did share a body and mind in combat as Firestorm, and there were some smaller things they craved that originally came from the other. But they were still largely different people who occasionally worked together.
Unnoticeable by the other members of the team, their minds were slowly melting together every time they transformed in their superhero alter-ego. They were intrinsically linked, a bond which could not even be severed by death, a link that is anchored deep within their beings. And as such, with each transformation, they gained another small piece of their partner.
They did not mind this. Stein had researched their predicament and concluded it wasn't harmful in the long run. However, neither of them took into account the small pieces of themselves they were losing in the process.
That's what happens when two minds clash. Each contributes to the shared end product, but what one contributes is an aspect of their personality that the other one loses. The losses are so small that neither of them realized what they were losing. These losses were also irretrievable; what they lose is what they will never see again.
Eventually, after a long time had passed, the only difference between Jax and the professor was their appearances. They spent all their time together, doing the things they both enjoyed and they never disagreed on anything anymore. Even their own names were lost to them, but they somehow still recognized them when the Legends referred to them.
Since it all happened so subtly and over the course of years, the other Legends had failed to recognize they had changed so drastically. Their situation had gone so far that they were no longer happy about their own connection – no, they needed to connect everyone to this mind, they wanted the world to share their joy.
With their collective knowledge, they managed to create a device which would help them share the connection. They first tested it on an unsuspicious Nate Heywood. Though there was no immediate reaction and he would never be able to bond with either half of Firestorm the way Jax and Stein still could, Nate did feel the connection established in his mind. He saw them no longer as two, but as one; he no longer saw himself as an outsider, but a lucky individual who got valuable insights in their minds; and he felt inclined to help them in their quest, which now had also become his quest, because two as one had become three as one, though the third party still needed to be properly assimilated. One by one, they added the Legends to the Firestorm collective, some joining more willingly under the false pretenses than the others.
After the Legends became one with Firestorm, they shifted their focus on Earth's population and they traveled back to the late summer of 2017. With all their experiences and mindsets added to the collective, they decided to establish their mind to those of world leaders and their staff. After they took care of that, they had their mind-melt device manufactured and spread across the globe – again, under false pretenses. People joined with each other, and their minds were later linked to Firestorm. Within two years, everyone on Earth was Firestorm with varying degrees of autonomy and personality. The more recent additions were still somewhat their own person, while the originals – the ones they used to call "Legends" – were completely gone. They were merely Firestorm.
That is the story of how humanity became hive-minded. Men and women reproduced regardless of preference and their children were linked to the collective from birth, becoming the new generation of Firestorm. The Thanagarian invasion came, but they suffered many losses fighting the Firestorm collective that they retreated and focused on invading Mars instead.
The Firestorm collective lasted until humanity's extinction.
