( Timeline ) Ancient Egypt of Midgard :
• The Kree :
More than five thousand years ago, a rogue faction of Kree set out on an intergalactic campaign to create an army of super soldiers, to help their empire win a war, that had gone on for generations.
They sent science teams to different planets, conducting experiments on various alien lifeforms, hoping to unlock genetic potential for heightened abilities. These teams met with failure, everywhere they went, until they arrived on Midgard.
The Kree had significant success experimenting on humans, to the point where they built a sprawling installation beneath what is now Puerto Rico.
By exposing the altered humans, to the mist exhaled by Terrigen Crystals, a genetic mutation known as Terrigenesis was triggered, allowing the subjects to attain unique superhuman abilities, becoming the first Inhumans
Word of the Reapers' experiments eventually reached the Kree Empire, who sent a delegation to Midgard to shut down the project, bringing with them a Monolith that they intended to use to banish the Inhumans to a distant planet, believing they should not be allowed to exist.
These plans were foiled, however, by the intervention of Alveus, the Inhuman created to command the others in the Reapers' army. Alveus led the Inhumans in an insurrection against the Kree, driving them from Midgard.
The other Inhumans grew fearful of Alveus' considerable power and ambition, and so allied with some of the humans Alveus planned to subjugate, banishing him through the Kree Monolith to Maveth. Alveus's followers, believing he was destined to rule the world, established a secret society with the sole purpose of bringing him back to Midgard and, in the meantime, destabilizing it so it would be ripe for takeover when he did return. Over the millennia, the society evolved, taking many different shapes and being called by many different names, eventually taking shape as the international paramilitary terrorist organization known as HYDRA. The society was eventually able to obtain the Monolith, but were unsuccessful at bringing Alveus back, despite numerous attempts.
The Kree's abandonment of their project did not mean the end of the Inhumans. The altered gene proved hereditary, with the first generation being able to pass Inhuman traits through generations of descendants.
The Kree's hasty abandonment of their work also meant numerous Terrigen Crystals, contained in Diviners (devices the Kree used to determine whether or not subjects had the Inhuman gene), remained to be discovered.
• Hive :
Alveus was a parasitic Inhuman capable of infesting and possessing the corpses of dead humans and controlling other Inhumans through a hive mind. Because of his great power, Alveus was eventually banished to his barren planet, leaving him trapped for centuries. His followers had then founded a secret society with the sole purpose of finally bringing him back onto the Midgard in order to conquer it. Over these centuries, that society evolved into HYDRA.
In modern times, and usually referred to by the literal translation of his name, Hive, Alveus possessed Grant Ward's dead body when he was killed by Phil Coulson on Maveth, and was able to return to Midgard once more. Working alongside Gideon Malick, Hive eventually recovered his true strength as he was granted a small army of the Inhumans by Malick, as Hive had soon taken the controlling power over HYDRA from Malick.
Seeking to transform the Midgard's population into Inhumans so they could live in peace, Hive and his small army attempted to use Kree blood to recreate their original experiments which he had endured thousands of years earlier, as they proceeded to kidnap Holden Radcliffe in order to use his genius to finish his plans.
But Hive's plans were then challenged by the continued efforts of S.H.I.E.L.D. and the Secret Warriors, who remained determined to defeat him. Hive attempted to complete his plan by detonating a warhead filled with Terrigen Mist into the Midgard's atmosphere, which would transform all of the Humans into Primitives that became loyal to him. However, he was defeated when Lincoln Campbell trapped himself and Hive inside of a Quinjet with the bomb, detonating it safely in space, and killing both.
• En Sabah Nur :
En Sabah Nur, translated as "The Seven Lights", was the first naturally born, Midgard humanoid, with super powers. A freak "Mutation" from the Inhumans altered genes.
Born in ancient Egypt thousands of years ago. Abandoned at birth, due to his monstrous appearance, he was found and raised by a ruthless nomadic warrior. Forced into slavery, his powers allowed him to turn on his masters and conquer all that stood in his way.
He championed an extreme philosophy of mutant supremacy and "survival of the fittest"; only the strong deserve to survive and weakness should be mercilessly destroyed.
Over the centuries, Apocalypse was worshipped as a god by various cultures around the world, enforcing his rule using his strike force known as the Horsemen of Apocalypse.
However, he was defeated by his own Horsemen, and sealed away in a dream state. That lasted for thousands of years.