The Necrons are an ancient xenos race that have existed in secret until recent history. They first emerged during the War in Heaven, where they created the C'tan and gave up their mortal bodies for a soulless machine existence. After the war, they disappeared into their tomb worlds and waited for the galaxy to repopulate itself; slowly they are reawakening and rebuilding their ancient empire.
The Necrons are inorganic and therefore cannot make use of the Warp; however, they possess the most advanced technology in the galaxy, and have the only known FTL technology that doesn't use the Warp. They are also extremely hard to kill as they use "reanimation protocols" to revive fallen troops.
The Necron were originally the Necrontyr, a race that of short-lived but technologically advanced organic people. The Necrontyr were the rivals of the Old Ones and the Eldar, and resented their Warp-based abilities and long lives. The Necrontyr then discovered gaseous creatures around their star, eating its radiation. The Necrontyr coaxed them to enter giant metallic bodies imbued with all the knowledge that they had, and became the C'Tan.
The C'Tan convinced the Silent King and his Pharons to allow the C'Tan to eat the Necrontyr's souls, and in exchange they would fashion them immortal bodies. They agreed, but discovered that the C'Tan could now control them, and only the elites of the Necrons now had any semblance of free will. Still hungry for souls, the C'Tan made war with the Old Ones, nearly wiping them out. Thus began the War in Heaven, which consumed the galaxy and corrupted the Warp into what it is today. The War came to an end when Cegorach and The Deceiver tricked the other C'Tan to start eating each other, until the C'Tan became severely weakened, and those still alive were broken into many shards.
The Necron disappeared into various tomb worlds to wait for the galaxy to repopulate and become unprepared for another assault. They have slowly started to reawaken, wiping out entire worlds, and various dynasties have emerged in the Eastern Fringe. But their full force has yet to be awakened.
While many of the C'Tan were destroyed or eaten during the War in Heaven, those that survived were broken into shards; while each shard lacks the full power of the original, they are not powerless and are fully capable of operating independently. Each shard seeks to reunite to regain their power.