The Necrons originally called the Necrontyr were a race of humanoid beings that lived on a planet that was battered with radiation due to the sun they were orbiting, as a result of this they lived extremely short life-spans and all their buildings were essentially tombs, one day however they encountered the Old Ones because of the old ones immortality they wanted the old ones to tell them the secret to immortality but the old ones didn’t see them as worthy, so the Necrons had decided to declare war on the Old Ones so that they could take it for themselves, However the war didn’t last very long so the Necrontyr returned to their home-world and had discovered living beings that lived of the radiation of their sun these gas entities were immortal and so the Necrontyr built massive living metal bodies for them and gave them the name C’tan and started to worship them as gods and try to solve the secret of immortality.
The C’tan however had other plans they told the leader of the Necrontyr that they would give them what they wanted and gathered all of the living metal on the planet and created bodies of living metal once created they put all of their soles into these empty shells. very few Necrontyre could remember their past lives and became mindless servants to the c’tan the necrontyr were no more and so were named, the Necrons.
The C’tan who were ancient enemies of the old ones used their new immortal slaves of undying metal and declared all out war on creation and all life and so the old ones had created the Crorks who names were shortened to the Orks so that the Orks would help fight the Necrons and summoned the Eldar to help them, and so began The War of The Gods. By the end of the war the Necron numbers were far lower and so they retreated to their tomb worlds and berried themselves only to awaken when technology was no longer specially made to combat them and would return to destroy all.
The old ones had never been heard from sins they were either wiped out or the last few survivors went into hiding.